feat: allow manual AI model names for custom providers

[FaiChou]

Summary

Adds a manual model name input in AI provider settings.

Some OpenAI-compatible providers do not expose a /models endpoint. This is common for a few third-party AI APIs and coding-plan providers. Before this change, TablePro only allowed selecting models fetched from the provider, so users could not configure those endpoints even when chat completions worked with a known model name.

Changes

• Add a model name text field in the AI provider detail sheet
• Keep the fetched model list as an optional picker when /models is available
• Trim whitespace before saving the selected or manually entered model
• Update AI Assistant docs and changelog

Verification

• Ran xcodebuild -project TablePro.xcodeproj -scheme TablePro -configuration Debug build -skipPackagePluginValidation
• Retried with CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO
• Build is blocked locally because Secrets.xcconfig is missing. The standard run is also blocked by missing Mac Development signing assets.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[FaiChou]

I have read the CLA Document and I hereby sign the CLA.

[chatgpt-codex-connector]

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[datlechin]

Thank you