Mark llmprovider experimental

[JonasHelming]

What it does

Mark AI-related preferences for Hugging Face, LlamaFile, and Vercel AI as experimental by adding the experimental tag to their preference schemas. This provides users with clear visual indicators in the settings UI that these features are still under active development and may be subject to changes.

Changes:

• Added tags: ['experimental'] to all Hugging Face preferences (API key, models)
• Added tags: ['experimental'] to LlamaFile preferences (llamafiles configuration)
• Added tags: ['experimental'] to all Vercel AI preferences (OpenAI API key, Anthropic API key, official models, custom models)

Depends on #16783

How to test

1. Open the Theia application
2. Navigate to File > Preferences > Settings (or use the command palette: "Preferences: Open Settings (UI)")
3. Search for the following preferences and verify each displays an "Experimental" tag:
   • Search for "hugging" → ai-features.huggingFace.apiKey and ai-features.huggingFace.models
   • Search for "llamafile" → ai-features.llamafile.llamafiles
   • Search for "vercel" → ai-features.vercelAi.openaiApiKey, ai-features.vercelAi.anthropicApiKey, ai-features.vercelAi.officialModels, and ai-features.vercelAi.customModels
4. Verify that the "Experimental" tag appears as a styled badge next to each preference name

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines