✨ feat(function_calling): implement FC-001 to FC-004 improvements

FC-001: thoughtSignature support for Gemini 3
- Add DUMMY_THOUGHT_SIGNATURE constant
- Implement ensure_thought_signature() function to inject signature into functionCall parts

FC-002: MCP response handling
- Add normalize_tool_response() for various tool response formats
- Handles dict, string, and array inputs with proper Gemini Struct conversion

FC-003: Tool choice conversion
- Add GeminiToolConfig dataclass for Gemini API toolConfig structure
- Implement convert_tool_choice() to map OpenAI tool_choice to Gemini format

FC-004: Type case normalization
- Add TYPE_MAP_UPPER and TYPE_MAP_LOWER constants
- Add configurable type_map property on SchemaConverter
- Add _normalize_type() method for schema type conversion

All changes are backwards compatible with sensible defaults.