fix: prevent duplicate text emission in Gemini provider (fixes #9822)

[roomote]

This PR attempts to address Issue #9822. Feedback and guidance are welcome!

Problem

The Gemini provider was emitting duplicate text when streaming responses. This occurred when the API response chunks contained both candidates with structured content AND a fallback text property with the same content, causing the text to be yielded twice.

Solution

Added a processedContent flag to track whether any content was yielded from the structured candidates processing. The fallback to chunk.text now only occurs when no content was processed from candidates, preventing duplicate text emission.

Changes

• Modified src/api/providers/gemini.ts to add duplicate prevention logic
• Added comprehensive test cases in src/api/providers/__tests__/gemini.spec.ts to verify the fix

Testing

• Added test case to verify no duplication when both candidates and text are present
• Added test case to verify fallback still works when only text property is present
• All existing tests continue to pass

Fixes #9822

[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found. The fix correctly prevents duplicate text emission by tracking whether content was processed from the structured candidates before falling back to the text property.

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