[ai] Use .json for unary tests instead of .txt (#6916)

Follow up to #6915

---------

Co-authored-by: David Motsonashvili <davidmotson@google.com>
Co-authored-by: David Motsonashvili <davidmotson@gmail.com>
Co-authored-by: Daymon <17409137+daymxn@users.noreply.github.com>

Author: 4 people

firebase-ai/src/test/java/com/google/firebase/ai/DevAPIUnarySnapshotTests.kt

@@ -35,7 +35,7 @@ internal class DevAPIUnarySnapshotTests {
 
   @Test
   fun `short reply`() =
-    goldenDevAPIUnaryFile("unary-success-basic-reply-short.txt") {
+    goldenDevAPIUnaryFile("unary-success-basic-reply-short.json") {
       withTimeout(testTimeout) {
         val response = model.generateContent("prompt")
 
@@ -47,7 +47,7 @@ internal class DevAPIUnarySnapshotTests {
 
   @Test
   fun `long reply`() =
-    goldenDevAPIUnaryFile("unary-success-basic-reply-long.txt") {
+    goldenDevAPIUnaryFile("unary-success-basic-reply-long.json") {
       withTimeout(testTimeout) {
         val response = model.generateContent("prompt")
 
@@ -85,14 +85,14 @@ internal class DevAPIUnarySnapshotTests {
 
   @Test
   fun `invalid api key`() =
-    goldenDevAPIUnaryFile("unary-failure-api-key.txt", HttpStatusCode.BadRequest) {
+    goldenDevAPIUnaryFile("unary-failure-api-key.json", HttpStatusCode.BadRequest) {
       withTimeout(testTimeout) {
         shouldThrow<InvalidAPIKeyException> { model.generateContent("prompt") }
       }
     }
   @Test
   fun `unknown model`() =
-    goldenDevAPIUnaryFile("unary-failure-unknown-model.txt", HttpStatusCode.NotFound) {
+    goldenDevAPIUnaryFile("unary-failure-unknown-model.json", HttpStatusCode.NotFound) {
       withTimeout(testTimeout) { shouldThrow<ServerException> { model.generateContent("prompt") } }
     }
 }