fix: sanitize tool parameter schemas for Gemini API compatibility

[roomote-v0]

Related GitHub Issue

Closes: #12202

Description

This PR attempts to address Issue #12202 where Gemini models return 400 INVALID_ARGUMENT errors.

The root cause is that the Gemini API's parametersJsonSchema field does not fully support certain standard JSON Schema constructs that our tool definitions use:

• type: ["string", "null"] (array-type for nullable params) -- used by execute_command, search_files, codebase_search, ask_followup_question, generate_image, new_task, run_slash_command, and skill tools
• additionalProperties: false -- used by most tool definitions

The fix adds a sanitizeSchemaForGemini() utility that recursively converts these constructs before passing them to the Gemini API:

• type: ["string", "null"] becomes type: "string", nullable: true
• additionalProperties is stripped from all schema levels
• Handles nested properties, items, and anyOf/oneOf/allOf

The sanitization is applied in GeminiHandler.createMessage() when building function declarations. Since VertexHandler extends GeminiHandler, Vertex is also covered.

Feedback and guidance are welcome.

Test Procedure

• Added 12 unit tests in src/api/transform/__tests__/gemini-schema.spec.ts covering:
  • Nullable type conversion (["string", "null"], ["number", "null"])
  • additionalProperties removal (top-level and nested)
  • Deeply nested schemas (array items, nested objects)
  • anyOf/oneOf/allOf handling
  • Edge cases (null input, type arrays without null, single string types)
  • Real-world schemas matching execute_command and ask_followup_question tools
• All 12 new tests pass
• All 46 existing Gemini/Vertex tests continue to pass
• Full lint and type-check pass

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This fix is defensive -- if the Gemini API already accepts these schemas (e.g. via newer SDK versions), the sanitization is a no-op for valid schemas. The conversion only modifies constructs known to be problematic.

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