Add native Kimi Moonshot AI support

[robertmonka]

🧢 Changes

• Adds Kimi K2.6 as a selectable AI model in desktop AI settings.
• Adds Moonshot/Kimi-specific request handling for both the desktop OpenAI client and the Rust but-llm OpenAI provider.
• Detects Kimi either from kimi-* model names or a moonshot.ai custom endpoint.
• Sends Kimi requests with thinking: { type: "disabled" }, uses max_tokens, and avoids OpenAI-only structured output parameters.
• Supports Kimi structured output through JSON mode by injecting the expected JSON schema into the prompt.
• Requires direct OpenAI-compatible credentials for Kimi instead of GitButler-proxied OpenAI credentials.
• Allows https://api.moonshot.ai in desktop/server CSP connect sources.
• Adds Rust tests for Kimi flavor detection and request body generation.

☕️ Reasoning

Kimi's Moonshot API is OpenAI-compatible, but it does not accept every request shape GitButler currently sends to OpenAI. In particular, Kimi needs max_tokens instead of max_completion_tokens, should have thinking disabled for short GitButler AI tasks, and uses JSON mode rather than OpenAI's structured output schema format.

This keeps the existing OpenAI behavior unchanged while routing Kimi models through a small Moonshot-specific path, so users can select kimi-k2.6 and use their Moonshot API key for commit messages, branch summaries, and other AI-generated text.

🧪 Testing

• Added unit coverage for Kimi API flavor detection.
• Added unit coverage for Kimi plain chat and structured JSON request bodies.
• Not run locally while updating this PR description.

[Byron]

I'm very sorry, but I have to close all of your PRs on the basis of them being fully automated coming from a profile that doesn't show much related history.

Thanks a lot for your understanding.