fix: handle GitHub Copilot gpt-4.1 model ID response bug

[roomote]

This PR fixes issue #6105 where GitHub Copilot's gpt-4.1 model returns only the model ID ("copilot/gpt-4.1") instead of generating actual AI responses.

Problem

When using the VS Code Language Model API with GitHub Copilot's gpt-4.1 model, users were seeing "copilot/gpt-4.1" as the response instead of actual AI-generated content. This appears to be a bug in either the VS Code Language Model API or the GitHub Copilot extension.

Solution

• Added debug logging to track responses from VS Code LM API for Copilot models
• Implemented detection logic to identify when the model ID is returned as chunk content
• Skip chunks that contain only the model ID and continue processing valid content
• Throw a descriptive error when the entire response consists only of the model ID
• Added comprehensive test coverage for all edge cases

Testing

• Added 3 new test cases specifically for this bug
• All existing tests continue to pass
• Verified that the workaround correctly handles:
  • Model ID followed by valid content (skips ID, returns content)
  • Only model ID returned (throws descriptive error)
  • Different variations of model ID patterns

Notes

This is a workaround for what appears to be an upstream bug in the VS Code Language Model API or GitHub Copilot extension. The fix ensures users get a clear error message instead of seeing the model ID as the response.

Fixes #6105

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Important

Fixes GitHub Copilot gpt-4.1 model bug by skipping model ID responses and throwing errors for invalid content in vscode-lm.ts.

• Behavior:
  • In vscode-lm.ts, added logic to detect and skip chunks containing only the model ID for GitHub Copilot's gpt-4.1 model.
  • Throws an error if the entire response consists only of the model ID.
• Testing:
  • Added test cases in vscode-lm.spec.ts to verify handling of model ID responses, including variations in model ID patterns.
• Logging:
  • Added debug logging in vscode-lm.ts to track responses and identify when model ID is returned as content.

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[beenycool]

il test the fixes tommorow

[daniel-lxs]

This could help debug but debug changes shouldn't be merged into main