feat: enhance OpenAI message conversion for Gemini compatibility

[LeeroyDing]

Gemini API is... weird, to say the least. Should fix #461

Note

This PR enhances OpenAI message conversion to handle Gemini API's strict compatibility requirements. The Gemini API uses an OpenAI-compatible shim but has several quirks that differ from standard OpenAI:

Key changes:

1. Added is_gemini parameter to convert_messages_to_openai() to conditionally handle Gemini-specific formatting
2. Assistant message content is now an empty string "" instead of null when tool calls are present (required by Gemini)
3. Tool calls now include an index field (0-based) in Gemini mode
4. Tool result messages now include the function name field for Gemini (in addition to tool_call_id)
5. Updated three call sites in SpacebotModel to pass the is_gemini flag based on ApiType
6. Added comprehensive tests for both Gemini-specific and standard OpenAI behavior

_{Written by Tembo for commit 01a4f9f. This will update automatically on new commits.}

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