fix: correct Gemini countTokens payload format to prevent premature context truncation

[roomote]

Summary

This PR attempts to address Issue #8113 by fixing the incorrect payload format sent to Gemini's countTokens method, which was causing premature context truncation in long-running conversations.

Problem

The Gemini token counting was failing because we were sending Part[] instead of the required Content[] format. This caused the SDK to return undefined for token counts, forcing a fallback to the generic tiktoken estimator that overestimates tokens by 1.5x, leading to premature truncation at ~90k-130k tokens instead of the expected ~250k.

Solution

• Fixed countTokens method in src/api/providers/gemini.ts to wrap the parts in a proper Content structure with user role
• The SDK now receives the correct format: [{ role: "user", parts: Part[] }]
• Maintained the existing fallback mechanism for resilience

Changes

• src/api/providers/gemini.ts: Updated countTokens to send correct Content[] format
• src/api/providers/tests/gemini.spec.ts: Added comprehensive tests for countTokens including:
  • Correct format validation
  • Multimodal content handling
  • Fallback scenarios when SDK returns undefined or throws errors
  • Empty content handling

Testing

• ✅ All existing tests pass
• ✅ Added 5 new test cases for countTokens method
• ✅ Verified the correct Content[] structure is sent to the SDK

Notes

• The fallback to tiktoken remains in place as a safety net
• This fix addresses the primary cause of premature truncation
• The optional context window configuration adjustment mentioned in the issue can be handled separately if needed

Fixes #8113

Feedback and guidance are welcome!

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Important

Fixes countTokens payload format in gemini.ts to prevent premature context truncation, with comprehensive tests added.

• Behavior:
  • Fixes countTokens in gemini.ts to send Content[] format with user role, preventing premature context truncation.
  • Maintains fallback to tiktoken estimator if SDK returns undefined or errors.
• Testing:
  • Adds tests in gemini.spec.ts for correct format, multimodal content, fallback scenarios, and empty content handling.
  • Verifies correct Content[] structure is sent to SDK.
• Misc:
  • Addresses Issue [BUG] Gemini: premature truncation due to incorrect countTokens payload and inflated fallback; consider contextWindow alignment #8113.

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[roomote]

Reviewed my own code. Found it surprisingly coherent for something I wrote 5 minutes ago.

[roomote]

Could we make this comment slightly more detailed? Something like:

Suggested change

	// The SDK expects Content[] format: [{ role: "user", parts: Part[] }]
	// Wrap the parts in a proper Content structure with user role
	// The SDK expects Content[] format: [{ role: "user", parts: Part[] }]
	// Previously we were sending Part[] directly, causing the SDK to return undefined

This would help future maintainers understand why this specific format is critical.

[roomote]

Consider adding a test case with tool_use/tool_result content blocks? The issue specifically mentions functionCall/functionResponse parts can cause problems with the wrong format. Would be good to ensure our fix handles those cases too.