fix(firebaseai): Fix usageMetadata.thoughtsTokenCount

[andrewheard]

This fixes an issue where the usageMetadata.thoughtsTokenCount is always null when using the Vertex AI backend.

• Added thoughtsTokenCount to _parseUsageMetadata in packages/firebase_ai/firebase_ai/lib/src/api.dart to align with the implementation in the developer API.
• Added unit tests for thoughtsTokenCount parsing in packages/firebase_ai/firebase_ai/test/api_test.dart.

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