Increase Firebase AI Logic unit test coverage

[paulb777]

This PR significantly improves the unit test coverage for the FirebaseAI Swift Package Manager target. The new tests were generated with the assistance of the Gemini CLI
and address several key areas that were previously under-tested.

Key Improvements:

• Chat Tests (ChatTests.swift):

  • Added tests for the non-streaming sendMessage method, including history updates.
  • Added error handling tests for sendMessageStream to ensure the chat history is not updated with partial responses on failure.
  • Added tests for initializing a Chat instance with a pre-existing history.

• GenerativeModel Tests:

  • Added dedicated failure tests for countTokens to both GenerativeModelGoogleAITests.swift and GenerativeModelVertexAITests.swift.
  • Added comprehensive tests for tool/function calling to the Google AI backend tests (GenerativeModelGoogleAITests.swift) to ensure parity with the Vertex AI backend.

• Data Model and Utility Tests:

  • Added tests for complex and nested scenarios in JSONValueTests.swift.
  • Added tests for more complex combinations of PartsRepresentable types, including mixed data and image content, and error handling for invalid images.
  • Added tests for Safety.swift to ensure robust decoding of SafetyRating with missing or unknown values.

#no-changelog

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[paulb777]

/gemini description

[paulb777]

/gemini review

[gemini-code-assist]

Hi @paulb777,

It looks like you tried to use /gemini description in your previous comment. That's not a recognized command. If you're looking for a summary of the pull request, please use /gemini summary.

I'm now processing your request for a code review as indicated by /gemini review.

[gemini-code-assist]

Code Review

The pull request introduces new unit tests covering chat functionality, JSON value handling, parts representation, and safety features. Some tests can be improved by avoiding force-unwrapping, using object comparison for JSON encoding, and using XCTExpectFailure for known test failures.

[andrewheard]

I just realized that all of the tests in this file are using the vertexai samples. We'd need to create similar samples for googleai and check them in.

[paulb777]

The stub files are missing, so I'll delete for now since there's nothing different about these tests than the existing vertexai ones.

[paulb777]

Thanks for the review. I asked Gemini CLI to address the comments. I couldn't find the mock response files either so I deleted GenerativeModelGoogleAITests+Coverage.swift for now. Gemini's summary below

✦ I have addressed all the review comments:

1. GenerativeModelGoogleAITests+Coverage.swift: I attempted to update the tests to use the correct googleai subdirectory for mock responses, but I was unable to locate the
   correct files. I have skipped this step.
2. PartsRepresentableTests+Coverage.swift: I have removed the !os(visionOS) condition to enable the tests on that platform.
3. SafetyTests.swift: I have updated the encoding tests to use JSONEncoder and verify the encoded JSON string.