MPT-17888: add first-level field annotations to Parameter and ParameterGroup models

Add type-annotated fields to the Parameter and ParameterGroup model
classes derived from the OpenAPI spec (ParameterDefinition and
ParameterGroup schemas respectively).

All fields are typed as T | None since RQL select can exclude any
field from the API response. Nested objects (audit, group, product,
constraints, options) are typed as BaseModel | None as no dedicated
sub-model classes exist for them yet.

Parameter fields: name, description, scope, phase, context, type,
status, external_id, display_order, group, product, constraints,
audit, options.

ParameterGroup fields: name, label, description, display_order,
default, parameter_count, product, audit.

Also extends unit tests for both models to cover field access,
nested BaseModel typing, and optional field absence.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: albertsola

mpt_api_client/resources/catalog/products_parameter_groups.py

@@ -6,10 +6,31 @@
     ManagedResourceMixin,
 )
 from mpt_api_client.models import Model
+from mpt_api_client.models.model import BaseModel
 
 
 class ParameterGroup(Model):
-    """Parameter Group resource."""
+    """Parameter Group resource.
+
+    Attributes:
+        name: Parameter group name.
+        label: Display label for the parameter group.
+        description: Parameter group description.
+        display_order: Display order of the group.
+        default: Whether this is the default parameter group.
+        parameter_count: Number of parameters in this group.
+        product: Reference to the product this group belongs to.
+        audit: Audit information (created, updated events).
+    """
+
+    name: str | None
+    label: str | None
+    description: str | None
+    display_order: int | None
+    default: bool | None
+    parameter_count: int | None
+    product: BaseModel | None
+    audit: BaseModel | None
 
 
 class ParameterGroupsServiceConfig:

mpt_api_client/resources/catalog/products_parameters.py

@@ -6,10 +6,43 @@
     ManagedResourceMixin,
 )
 from mpt_api_client.models import Model
+from mpt_api_client.models.model import BaseModel
 
 
 class Parameter(Model):
-    """Parameter resource."""
+    """Parameter resource.
+
+    Attributes:
+        name: Parameter name.
+        description: Parameter description.
+        scope: Parameter scope (e.g. Agreement, Item, Request, Subscription, Order, Asset).
+        phase: Parameter phase (e.g. Configuration, Order, Fulfillment).
+        context: Parameter context (e.g. None, Purchase, Change, Configuration, Termination).
+        type: Parameter type (e.g. SingleLineText, MultiLineText, Address, etc.).
+        status: Parameter status.
+        external_id: External identifier for the parameter.
+        display_order: Display order of the parameter.
+        group: Reference to the parameter group.
+        product: Reference to the product this parameter belongs to.
+        constraints: Parameter constraints (required, hidden, readonly).
+        audit: Audit information (created, updated events).
+        options: Type-specific parameter options.
+    """
+
+    name: str | None
+    description: str | None
+    scope: str | None
+    phase: str | None
+    context: str | None
+    type: str | None
+    status: str | None
+    external_id: str | None
+    display_order: int | None
+    group: BaseModel | None
+    product: BaseModel | None
+    constraints: BaseModel | None
+    audit: BaseModel | None
+    options: BaseModel | None
 
 
 class ParametersServiceConfig: