Add Gemini default_api recovery example

[gold-silver-copper]

Summary

• add a runnable Gemini example that steers the model toward the legacy default_api workspace runtime name
• attach an InvalidToolCallHook that repairs default_api to the registered JavaScript tool
• print per-attempt diagnostics and exit with an error if no attempt exercises the repaired default_api path

Verification

• cargo check --example gemini_default_api_recovery --features derive
• RIG_GEMINI_DEFAULT_API_CANARY_ATTEMPTS=6 cargo run --example gemini_default_api_recovery --features derive