feat(gemini): add namespace-aware OpenAI Responses tool resolution and custom tool call conversion

- Add shared tool descriptor collection and winner selection for Responses tools (top-level vs `additional_tools`, direct vs namespace child, and ordering rules).
- Introduce sanitized Gemini function name mapping with collision disambiguation and 64-char-safe truncation.
- Build forward/reverse tool identity maps for restoring original tool identity (`name`, `namespace`, `custom`) during translation.
- Update Gemini→Responses streaming conversion to emit proper custom tool call events and identity-aware function call events.
- Add helpers for translating `tool_choice` to Gemini config and unwrapping custom tool input payloads.

Closes: #5088