[Vertex AI] Add countTokens support for Developer API via VinF

[andrewheard]

Added support for countTokens using the Developer API through the Vertex AI in Firebase (firebasevertexai.googleapis.com) backend. This combination is an edge case where the model resource name (e.g., projects/my-project/models/gemini-2.0-flash) in the countTokens request URL differs from the resource name in the request field CountTokensRequest.generateContentRequest.name. The integration tests have been updated to perform a countTokens request using both firebasevertexai.googleapis.com (Vertex AI and Developer API) and generativelanguage.googleapis.com (Developer API only) to ensure no regressions.

#no-changelog

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[andrewheard]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds support for countTokens using the Developer API through the Vertex AI in Firebase backend. The changes include modifications to the GenerativeModel, VertexAI, and CountTokensRequest classes to handle the different model resource name formats required by the various backends. Integration tests have also been updated to ensure no regressions. Overall, the changes seem well-structured and address the specific edge case described in the pull request description.

Summary of Findings

• Model Name Handling: The introduction of the modelName property in GenerativeModel and its usage in constructing the generateContentRequestModelResourceName seems like a good approach to handle the different model name formats. However, it's crucial to ensure that this new property is always correctly initialized and used consistently throughout the codebase.
• Code Duplication: The developerModelResourceName function in VertexAI.swift has similar logic for .firebaseVertexAIStaging and .firebaseVertexAIProd. Consider refactoring to reduce code duplication.
• Test Coverage: The integration tests have been updated, but it's important to ensure that all edge cases and different configurations are thoroughly tested to prevent regressions.
• Documentation: The documentation for the modelResourceName parameter in the GenerativeModel initializer is comprehensive and helpful. Ensure that all public APIs are well-documented.

Merge Readiness

The pull request introduces important functionality for handling a specific edge case with the countTokens API. The code appears to be well-structured and addresses the issue effectively. However, before merging, it's recommended to address the potential code duplication in developerModelResourceName and ensure comprehensive test coverage for all configurations. I am unable to directly approve this pull request, and other reviewers should review and approve this code before merging.

[gemini-code-assist]

Consider adding a comment here to explain why modelResourceName is passed here, as opposed to generateContentRequestModelResourceName.

[gemini-code-assist]

Consider adding a comment explaining why developerV1Spark is used here, and what the expected behavior is.

[andrewheard]

For reference in this PR review, developerV1 and developerV1Beta now use the Vertex AI in Firebase backend, which doesn't support the Developer API on v1. developerV1Spark talks directly to the Generative Language backend.

[paulb777]

Lines 64 and 65 are confusing to me. Let's discuss.