Fixes #4514 by sanitizing unsigned thinking signatures for Claude target models in OpenAI Chat Completions requests to Antigravity. - Export SanitizeAntigravityClaudeGeminiRequestSignatures in gemini request translator and call it for Claude target models in OpenAI Chat Completions requests. - Use json.Decoder with UseNumber() in SanitizeAntigravityClaudeGeminiRequestSignatures to preserve exact JSON number precision (preventing float64 truncation of large integers and high-precision numbers during unmarshal/marshal). - Add unit tests verifying unsigned reasoning_content sanitization, empty turn dropping, exact number precision preservation, and functionCall signature stripping for Claude target models.