fix(translator): restore usageMetadata in Gemini responses from Antigravity

[sususu98]

Summary

Fixes #1297

When using Gemini API format with Antigravity backend, the response incorrectly contains cpaUsageMetadata instead of the standard usageMetadata field.

Changes

• Add restoreUsageMetadata() function to rename cpaUsageMetadata back to usageMetadata
• Apply restoration in both streaming (ConvertAntigravityResponseToGemini) and non-streaming (ConvertAntigravityResponseToGeminiNonStream) responses
• Add unit tests for the fix

Root Cause

In #747, StripUsageMetadataFromJSON was introduced to rename usageMetadata to cpaUsageMetadata in non-terminal chunks. This was designed for the Claude translator to consume, but the Gemini translator was not updated to restore the original field name before returning responses to clients.

Testing

go test ./internal/translator/antigravity/gemini/... -v

All tests pass including new tests for restoreUsageMetadata.

[gemini-code-assist]

Summary of Changes

Hello @sususu98, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where Gemini API responses, when processed through the Antigravity backend, incorrectly contained cpaUsageMetadata instead of the standard usageMetadata field. The change ensures that all Gemini responses now correctly expose usageMetadata, aligning with the expected API format and improving client compatibility.

Highlights

• Usage Metadata Restoration: Introduced a new restoreUsageMetadata() function to correctly rename cpaUsageMetadata back to usageMetadata in Gemini API responses.
• API Response Consistency: Applied the restoreUsageMetadata() function to both streaming (ConvertAntigravityResponseToGemini) and non-streaming (ConvertAntigravityResponseToGeminiNonStream) Gemini responses to ensure consistent output.
• Test Coverage: Added comprehensive unit tests for the new restoreUsageMetadata() function and its integration into the Gemini response conversion logic.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where cpaUsageMetadata was being returned instead of the expected usageMetadata. The solution, which involves adding a restoreUsageMetadata function and applying it in both streaming and non-streaming response handlers, is well-implemented. The accompanying unit tests are thorough and cover the new logic effectively. I have one suggestion to improve the error handling in the new restoreUsageMetadata function to make it more robust against potential data inconsistencies.

[gemini-code-assist]

Ignoring errors from sjson operations can lead to returning a partially modified JSON object. If SetRawBytes succeeds but DeleteBytes fails, the resulting chunk will contain both usageMetadata and cpaUsageMetadata. It's safer to handle these errors and return the original chunk on failure to ensure the rename operation is atomic.

                res, err := sjson.SetRawBytes(chunk, "usageMetadata", []byte(cpaUsage.Raw))
                if err != nil {
                        return chunk
                }
                res, err = sjson.DeleteBytes(res, "cpaUsageMetadata")
                if err != nil {
                        return chunk
                }
                chunk = res