⚙️ config: add FC-001 and FC-004 function calling improvement settings

Add configuration options for function calling improvements:
- FUNCTION_CALLING_THOUGHT_SIGNATURE: Enable thoughtSignature support for Gemini 3 compatibility
- FUNCTION_CALLING_UPPERCASE_TYPES: Enable UPPERCASE JSON Schema types (pending UI verification)

Both settings are documented in .env.example with default values that maintain backwards compatibility.