Consolidate MoSAPI models and enhance null-safety

- Moves model records into a single MosApiModels.java file.
- Switches to ImmutableList/ImmutableMap with non-null defaults in constructors.
- Removes redundant pass-through methods in MosApiStateService.
- Updates tests to use Java Text Blocks and non-null collection assertions.

Author: njshah301

…ry/mosapi/model/MosApiErrorResponse.java …registry/mosapi/MosApiErrorResponse.javacore/src/main/java/google/registry/mosapi/model/MosApiErrorResponse.java renamed to core/src/main/java/google/registry/mosapi/MosApiErrorResponse.java core/src/main/java/google/registry/mosapi/model/MosApiErrorResponse.java renamed to core/src/main/java/google/registry/mosapi/MosApiErrorResponse.java

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package google.registry.mosapi.model;
+package google.registry.mosapi;
 
 /**
  * Represents the generic JSON error response from the MoSAPI service for a 400 Bad Request.

core/src/main/java/google/registry/mosapi/MosApiException.java

@@ -17,7 +17,6 @@
 import static java.lang.annotation.ElementType.TYPE;
 import static java.lang.annotation.RetentionPolicy.RUNTIME;
 
-import google.registry.mosapi.model.MosApiErrorResponse;
 import java.io.IOException;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Retention;

core/src/main/java/google/registry/mosapi/MosApiModels.java

@@ -0,0 +1,122 @@
+// Copyright 2025 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.mosapi;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.gson.annotations.Expose;
+import com.google.gson.annotations.SerializedName;
+import java.util.List;
+import java.util.Map;
+import javax.annotation.Nullable;
+
+/** Data models for ICANN MoSAPI. */
+public final class MosApiModels {
+
+  private MosApiModels() {}
+
+  /**
+   * A wrapper response containing the state summaries of all monitored services.
+   *
+   * <p>This corresponds to the collection of service statuses returned when monitoring the state of
+   * a TLD
+   *
+   * @see <a href="https://www.icann.org/mosapi-specification.pdf">ICANN MoSAPI Specification,
+   *     Section 5.1</a>
+   */
+  public record AllServicesStateResponse(
+      // A list of state summaries for each monitored service (e.g. DNS, RDDS, etc.)
+      @Expose @SerializedName("serviceStates") List<ServiceStateSummary> serviceStates) {
+
+    public AllServicesStateResponse {
+      serviceStates = (serviceStates == null) ? ImmutableList.of() : serviceStates;
+    }
+  }
+
+  /**
+   * A summary of a service incident.
+   *
+   * @see <a href="https://www.icann.org/mosapi-specification.pdf">ICANN MoSAPI Specification,
+   *     Section 5.1</a>
+   */
+  public record IncidentSummary(
+      @Expose @SerializedName("incidentID") String incidentID,
+      @Expose @SerializedName("startTime") long startTime,
+      @Expose @SerializedName("falsePositive") boolean falsePositive,
+      @Expose @SerializedName("state") String state,
+      @Expose @SerializedName("endTime") @Nullable Long endTime) {}
+
+  /**
+   * A curated summary of the service state for a TLD.
+   *
+   * <p>This class aggregates the high-level status of a TLD and details of any active incidents
+   * affecting specific services (like DNS or RDDS), based on the data structures defined in the
+   * MoSAPI specification.
+   *
+   * @see <a href="https://www.icann.org/mosapi-specification.pdf">ICANN MoSAPI Specification,
+   *     Section 5.1</a>
+   */
+  public record ServiceStateSummary(
+      @Expose @SerializedName("tld") String tld,
+      @Expose @SerializedName("overallStatus") String overallStatus,
+      @Expose @SerializedName("activeIncidents") List<ServiceStatus> activeIncidents) {
+
+    public ServiceStateSummary {
+      activeIncidents = activeIncidents == null ? ImmutableList.of() : activeIncidents;
+    }
+  }
+
+  /** Represents the status of a single monitored service. */
+  public record ServiceStatus(
+      /**
+       * A JSON string that contains the status of the Service as seen from the monitoring system.
+       * Possible values include "Up", "Down", "Disabled", "UP-inconclusive-no-data", etc.
+       */
+      @Expose @SerializedName("status") String status,
+
+      //  A JSON number that contains the current percentage of the Emergency Threshold
+      //  of the Service. A value of "0" specifies that there are no Incidents
+      //  affecting the threshold.
+      @Expose @SerializedName("emergencyThreshold") double emergencyThreshold,
+      @Expose @SerializedName("incidents") List<IncidentSummary> incidents) {
+
+    public ServiceStatus {
+      incidents = incidents == null ? ImmutableList.of() : incidents;
+    }
+  }
+
+  /**
+   * Represents the overall health of all monitored services for a TLD.
+   *
+   * @see <a href="https://www.icann.org/mosapi-specification.pdf">ICANN MoSAPI Specification,
+   *     Section 5.1</a>
+   */
+  public record TldServiceState(
+      @Expose @SerializedName("tld") String tld,
+      long lastUpdateApiDatabase,
+
+      // A JSON string that contains the status of the TLD as seen from the monitoring system
+      @Expose @SerializedName("status") String status,
+
+      // A JSON object containing detailed information for each potential monitored service (i.e.,
+      // DNS,
+      //  RDDS, EPP, DNSSEC, RDAP).
+      @Expose @SerializedName("testedServices") Map<String, ServiceStatus> serviceStatuses) {
+
+    public TldServiceState {
+      serviceStatuses = (serviceStatuses == null) ? ImmutableMap.of() : serviceStatuses;
+    }
+  }
+}

core/src/main/java/google/registry/mosapi/MosApiStateService.java

@@ -20,13 +20,12 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.common.flogger.FluentLogger;
 import google.registry.config.RegistryConfig.Config;
-import google.registry.mosapi.model.AllServicesStateResponse;
-import google.registry.mosapi.model.ServiceStateSummary;
-import google.registry.mosapi.model.ServiceStatus;
-import google.registry.mosapi.model.TldServiceState;
+import google.registry.mosapi.MosApiModels.AllServicesStateResponse;
+import google.registry.mosapi.MosApiModels.ServiceStateSummary;
+import google.registry.mosapi.MosApiModels.ServiceStatus;
+import google.registry.mosapi.MosApiModels.TldServiceState;
 import jakarta.inject.Inject;
 import jakarta.inject.Named;
-import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 
@@ -52,14 +51,10 @@ public MosApiStateService(
     this.tldExecutor = tldExecutor;
   }
 
-  /** Shared internal logic to fetch raw data from ICANN MoSAPI state monitoring. */
-  private TldServiceState fetchRawState(String tld) throws MosApiException {
-    return serviceMonitoringClient.getTldServiceState(tld);
-  }
-
   /** Fetches and transforms the service state for a given TLD into a summary. */
   public ServiceStateSummary getServiceStateSummary(String tld) throws MosApiException {
-    return transformToSummary(fetchRawState(tld));
+    TldServiceState rawState = serviceMonitoringClient.getTldServiceState(tld);
+    return transformToSummary(rawState);
   }
 
   /** Fetches and transforms the service state for all configured TLDs. */
@@ -91,7 +86,7 @@ public AllServicesStateResponse getAllServiceStateSummaries() {
   }
 
   private ServiceStateSummary transformToSummary(TldServiceState rawState) {
-    List<ServiceStatus> activeIncidents = null;
+    ImmutableList<ServiceStatus> activeIncidents = ImmutableList.of();
     if (DOWN_STATUS.equalsIgnoreCase(rawState.status())) {
       activeIncidents =
           rawState.serviceStatuses().entrySet().stream()

core/src/main/java/google/registry/mosapi/ServiceMonitoringClient.java

@@ -17,8 +17,7 @@
 import com.google.gson.Gson;
 import com.google.gson.JsonIOException;
 import com.google.gson.JsonSyntaxException;
-import google.registry.mosapi.model.MosApiErrorResponse;
-import google.registry.mosapi.model.TldServiceState;
+import google.registry.mosapi.MosApiModels.TldServiceState;
 import jakarta.inject.Inject;
 import java.util.Collections;
 import okhttp3.Response;