[codex] Add Gemini default_api canary tests

[gold-silver-copper]

Summary

Adds an ignored live Gemini canary for a default_api tool-call namespace leak in Gemini streaming tool flows.

What Changed

• Adds tests/providers/gemini/live/default_api.rs with a workspace-like Gemini streaming tool flow.
• Wires the new live test into tests/providers/gemini/mod.rs.
• Uses the same broad shape as the reported issue: gemini-3.1-pro-preview, thinking config, a declared JavaScript tool, a JavaScriptProgram schema via schema_for!, an ExecutorResponse(Result<Value, String>), .default_max_turns(...), .with_history(...), and stream consumption that records reasoning, tool calls, tool results, usage, and final text.
• Keeps the fixture intentionally generic so the public test does not expose private application details.

Notes

The live canary is ignored by default because it requires GEMINI_API_KEY and calls the live Gemini API. When run manually, it can expose the namespace leak in Gemini reasoning text, e.g. default_api:JavaScript, even when the structured tool call is correctly named JavaScript.

Validation

• cargo fmt
• cargo test --test gemini streaming_javascript_tool_does_not_surface_default_api_or_empty_final_response
• git diff --check