[codex] fix Gemini default_api tool calls

[gold-silver-copper]

Summary

Fixes Gemini tool-call handling around malformed default_api outputs and Gemini 3 function-call correlation.

• Preserve Gemini functionCall.id through Rig ToolCall.call_id and send matching functionResponse.id back in history.
• Validate Gemini structured function calls against declared function names before emitting Rig tool calls.
• Normalize only safe default_api.<declared_tool> / default_api:<declared_tool> wrappers, while rejecting bare default_api, unknown tools, markdown tool_code leakage, and unexpected code-execution parts.
• Treat unknown tool dispatch as a fatal protocol/tool error instead of converting ToolNotFoundError into a successful tool-result string.
• Add deterministic regression tests and a Gemini 3 cassette test that verifies streamed tool calls preserve thought signatures.

Root Cause

Gemini can sometimes emit malformed or non-contractual tool-use output involving default_api. Rig was allowing unknown tool calls to reach the tool server, then stringifying ToolNotFoundError into a normal tool result, which could poison the follow-up turn and end with a confusing empty response.

Validation

• cargo fmt --check
• cargo test -p rig-core prompt_request
• cargo test -p rig-core gemini
• cargo test -p rig-core --lib
• cargo clippy -p rig-core --all-targets --all-features
• cargo test --test gemini
• git diff --check

Note: cargo test -p rig-core runs the lib tests successfully but still fails in an unrelated existing Anthropic doctest that imports rig::... from inside rig-core.