[Firebase AI] Add GenerativeModel tests using Dev API mock responses

[andrewheard]

Split GenerativeModelTests into GenerativeModelGoogleAITests and GenerativeModelVertexAITests to test against the two sets of mock responses for the Google AI and Vertex AI backends.

#no-changelog

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[gemini-code-assist]

Code Review

This pull request introduces new tests for GenerativeModel using mock responses for both Google AI and Vertex AI backends. The code is well-structured and the tests cover various success and failure scenarios. However, there are a few areas that could be improved for better maintainability and clarity.

Summary of Findings

• Duplicated httpRequestHandler and nonHTTPRequestHandler functions: The httpRequestHandler and nonHTTPRequestHandler functions are duplicated in GenerativeModelVertexAITests.swift and were moved to GenerativeModelTestUtil.swift. Ensure that all call sites are updated to use the utility functions to avoid future duplication.
• Inconsistent use of GenerativeModelTestUtil.testFirebaseInfo(): The testFirebaseInfo() function was previously defined in GenerativeModelVertexAITests.swift and is now available in GenerativeModelTestUtil.swift. Ensure that all call sites are updated to use the utility function to avoid future duplication.
• Incomplete test coverage for error cases: While the tests cover many scenarios, consider adding more specific tests for different error conditions, especially around safety ratings and prompt blocking, to ensure comprehensive coverage.

Merge Readiness

The pull request is almost ready for merging. Addressing the duplicated functions and ensuring consistent use of utility functions will improve maintainability. While I am unable to directly approve this pull request, I recommend that the author address the comments before merging, and that other reviewers also review and approve this code before merging.