feat(ai): add GitHub Copilot language model integration

[planger]

What it does

Introduces a new @theia/ai-copilot package that integrates GitHub Copilot as a language model provider in Theia's AI framework. This allows users to authenticate with their GitHub Copilot subscription and use Copilot models (e.g., GPT-4o, Claude 3.5 Sonnet) through Theia's AI features.

Therefore, we add:

• OAuth device flow authentication with GitHub
• Language model implementation using Copilot's chat completions API
• Status bar indicator showing authentication state
• Commands for login/logout and model management
• Preferences for configuring available Copilot models

How to test

1. Build and start the browser example: npm run build:browser && npm run start:browser
2. Check the status bar - you should see a "Copilot" indicator showing "Not signed in"
3. Click the status bar item or run command "Copilot: Sign In"
4. A dialog appears with a device code - click the link to open GitHub's device authorization page
5. Enter the code and authorize the application
6. The dialog should update to show "Authenticated" and the status bar should reflect the signed-in state
7. Open AI agent configuration - Copilot models should now be available as language models for your agent
8. Test the models by using any AI feature (e.g., AI chat) with a Copilot model selected

Note: The OAuth flow currently uses a GitHub App registered in the EclipseSource organization. We need this GitHub App just for authentication, no access is required or resources are shared. Before merging, this should be changed to an app owned by eclipse-theia under the Eclipse Foundation. This PR is primarily to get the discussion rolling and enable testing of the draft implementation.

Note: Obviously this will only work, if your Github user has access to the Copilot features (i.e. an active Copilot suscription).

Follow-ups

• Register GitHub App under Eclipse Foundation / eclipse-theia organization
• Add support for Copilot-specific features (e.g., code completions API)
• Add more comprehensive error handling for API rate limits

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

[github-actions]

New @theia Package(s) Detected

This PR adds the following new package(s): ai-copilot

Please ensure the following checklist items are completed before merging:

• package.json contains all required fields (name, version, description, license, etc.) and scripts
• README.md was added (please align with the existing README structure, e.g. README.md)
• Config files (tsconfig.json, .eslintrc.js) were added and align with the existing packages
• Folder structure follows Theia conventions (see Code Organization)
• Package is added to the example applications (i.e. browser, browser-only, electron)
• New packages must be published manually by a Theia committer initially (see also Release Process - Newly added Theia packages).
  If you are not a committer or do not have enough time, please open a follow-up ticket with the label toDoWithRelease to inform the release team about the new package (see for example: Publish new package @theia/ai-codex intially to npm #16651).
• If the package should also be part of the Theia IDE, please open a ticket for the Theia IDE
  and assign the toDoWithRelease (see for example: Add extension @theia/terminal-manager to the IDE theia-ide#615)