[Firebase AI] Add Candidate decoding tests for urlMetadata

[andrewheard]

• Reverted the changes to MockURLProtocol since GenerativeModelGoogleAITests and GenerativeModelVertexAITests are specifically meant for mock response testing.
• Added a similar decoding test in GenerateContentResponseTests to verify the same parsing logic when urlMetadata is [] in the Candidate.
• Added another decoding test in GenerateContentResponseTests to verify that the same behaviour occurs when urlMetadata is omitted (rather than an empty array).
  • This behaviour is more likely to be observed in practice since [] is typically omitted during JSON serialization in the backend (since it is a default value).

#no-changelog

[gemini-code-assist]

Code Review

This is a solid pull request that improves the test suite's structure and maintainability. Moving the urlMetadata decoding tests to GenerateContentResponseTests makes them more focused and correctly tests the Decodable implementation in isolation. The simplification of MockURLProtocol by removing the dataRequestHandler is also a welcome cleanup.

I have one minor suggestion in GenerateContentResponseTests.swift to reduce some code duplication in the newly added tests. Overall, great work!

[paulb777]

Thanks for cleaning this up! LGTM on green