[Vertex AI] Add APIConfig to userInfo dictionary in coders

[andrewheard]

Added the APIConfig to the JSONEncoder and JSONDecoder used in GenerativeAIService. The APIConfig is accessible via the computed property apiConfig during encoding and decoding of request / response types. This will simplify future decoding of CitationMetadata, which is deeply nested in the GenerateContentResponse, based on the backend API in use.

#no-changelog

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[gemini-code-assist]

Code Review

This pull request introduces the APIConfig to the JSONEncoder and JSONDecoder used in GenerativeAIService. This allows the APIConfig to be accessible during encoding and decoding of request/response types, which simplifies future decoding of CitationMetadata. The changes look good overall, with a few minor suggestions for improvement.

Summary of Findings

• Force-unwrapping URLs: The code uses force-unwrapping when creating URLs, which can lead to runtime crashes if the URL string is invalid. Consider using optional binding or URL(string:) to handle potential nil values gracefully.
• Fatal errors: The code uses fatalError in several places when the APIConfig is not found in the coder's userInfo. While this is acceptable for development, consider using a more graceful error handling mechanism in production code.
• Redundant decoder configuration: The code initializes a JSONDecoder with keyDecodingStrategy = .convertFromSnakeCase which is no longer needed since the APIConfig is now being passed in.

Merge Readiness

The pull request is almost ready for merging. Before merging, I recommend addressing the force-unwrapping of URLs and considering a more graceful error handling mechanism than fatalError. I am unable to directly approve this pull request, and users should have others review and approve this code before merging.

[andrewheard]

@paulb777 I thought about this a bit more and realized that it would be a breaking change for anyone that is using our public Codable conformances (e.g., to read/write Chat history to/from disk). I think I'll switch to a simpler approach to decode CitationMetadata based on the keys in the payload (citations vs. citationSources). We can re-evaluate if there are bigger differences we need to handle in the future.