Prepare VertexAI SDK for release

[daymxn]

Per b/414829218,

This ensures the Vertex AI SDK is properly aligned for the upcoming release. More specifically, this PR does the following:

• Adds deprecation notices for all the public classes and interfaces, besides the root FirebaseVertexAI class and the relevant extension methods.
• Updates the golden file version.
• Adds the change from [Ai] Add workaround for invalid SafetyRating from the backend. #6925 for working around invalid SafetyRating(s).
• Adds a changelog entry that aligns with the deprecation notices.
• Adds the changes from Add headers to be able to track bidi usage  #6939 for tracking bidi usage.
• Adds the changes from Align LiveSeverMessage related protos #6910 for aligning the bidi protos.

[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (548dc28) and merge commit (6b8bd18).

Test Logs

• Base (548dc28): https://github.com/firebase/firebase-android-sdk/actions/runs/14875183796
• Merge (6b8bd18): https://github.com/firebase/firebase-android-sdk/actions/runs/14892398192

1. https://storage.googleapis.com/firebase-sdk-metric-reports/jL20DwXbeO.html

[github-actions]

Test Results

 18 files   -  24   18 suites   - 24   38s ⏱️ -44s
 88 tests  - 236   88 ✅  - 236  0 💤 ±0  0 ❌ ±0 
176 runs   - 484  176 ✅  - 484  0 💤 ±0  0 ❌ ±0 

Results for commit 5518099. ± Comparison against base commit 548dc28.

This pull request removes 324 and adds 88 tests. Note that renamed tests count towards both.

com.google.firebase.remoteconfig.ConfigTests ‑ Custom Signals builder support multiple types
com.google.firebase.remoteconfig.ConfigTests ‑ Firebase#remoteConfig should delegate to FirebaseRemoteConfig#getInstance()
com.google.firebase.remoteconfig.ConfigTests ‑ Firebase#remoteConfig should delegate to FirebaseRemoteConfig#getInstance(FirebaseApp, region)
com.google.firebase.remoteconfig.ConfigTests ‑ FirebaseRemoteConfigSettings builder works
com.google.firebase.remoteconfig.ConfigTests ‑ Overloaded get() operator returns default value when key doesn't exist
com.google.firebase.remoteconfig.ConfigTests ‑ Overloaded get() operator returns value when key exists
com.google.firebase.remoteconfig.CustomSignalsTest ‑ testCustomSignals_builderPutDouble
com.google.firebase.remoteconfig.CustomSignalsTest ‑ testCustomSignals_builderPutDuplicateKeys
com.google.firebase.remoteconfig.CustomSignalsTest ‑ testCustomSignals_builderPutLong
com.google.firebase.remoteconfig.CustomSignalsTest ‑ testCustomSignals_builderPutMixedTypes
…

com.google.firebase.vertexai.GenerativeModelTesting ‑ exception thrown when using invalid location
com.google.firebase.vertexai.GenerativeModelTesting ‑ system calling in request
com.google.firebase.vertexai.SchemaTests ‑ basic schema declaration
com.google.firebase.vertexai.SchemaTests ‑ full schema declaration
com.google.firebase.vertexai.SerializationTests ‑ test Candidate serialization as Json
com.google.firebase.vertexai.SerializationTests ‑ test GenerateContentResponse serialization as Json
com.google.firebase.vertexai.SerializationTests ‑ test Schema serialization as Json
com.google.firebase.vertexai.SerializationTests ‑ test countTokensResponse serialization as Json
com.google.firebase.vertexai.SerializationTests ‑ test modalityTokenCount serialization as Json
com.google.firebase.vertexai.VertexAIStreamingSnapshotTests ‑ citation parsed correctly
…

♻️ This comment has been updated with latest results.

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-vertexai

  Type	Base (548dc28)	Merge (6b8bd18)	Diff
  aar	800 kB	833 kB	+33.3 kB (+4.2%)
  apk (aggressive)	1.54 MB	1.55 MB	+7.37 kB (+0.5%)
  apk (release)	9.51 MB	9.52 MB	+10.9 kB (+0.1%)

Test Logs

• Base (548dc28): https://github.com/firebase/firebase-android-sdk/actions/runs/14875183796
• Merge (6b8bd18): https://github.com/firebase/firebase-android-sdk/actions/runs/14892398192

1. https://storage.googleapis.com/firebase-sdk-metric-reports/rIl81g4M0v.html