decouple copilot completions from copilot chat (#14764)

[sharon-wang]

Cherry-picks #14764 to main. Resolved some merge conflicts arising from changed upstream completions and auth related code, as well as some minor test fixes.

Closes #14369

• ensures copilot completions are properly gated behind ai.enabled
• confirms copilot completions are still enabled even when chat.disableAIFeatures is true
• confirms copilot completions are disabled only with ai.enabled = false or github.copilot.enable = { "*": false }, or when github copilot is not auth'd with the provider modal
• updates the chat.disableAIFeatures setting description to be more clear how it affects copilot chat and completions
  • 
• keep status bar icon visible by default, but don't show chat info unless copilot chat is enabled
  • completions available and enabled
    • 
  • not auth'd with github
    • 
  • chat.disableAIFeatures = false + not auth'd with github
    • 
  • chat.disableAIFeatures = false + auth'd with github
    • 
• status bar icon updates when ai.enabled and chat.disableAIFeatures change, without having to reload
• isCompletionsOnlyMode indicates the default state where chat.disableAIFeatures = true, so only completions are applicable to the chat status
• adds vitest coverage for isCompletionsOnlyMode (ai.enabled off, chat.disableAIFeatures = true, chat.disableAIFeatures = false) and for the ai.enabled gate in isCompletionsEnabled
• adds a regression check that flipping chat.disableAIFeatures doesn't change inline completions
• to use github copilot in positron for AI features, you must authenticate using the provider modal

Release Notes

Bug Fixes

• Show GitHub Copilot completions status icon by default (Assistant: keep copilot completions enabled when copilot chat is disabled #14369)
• Using GitHub account for AI features requires additional opt-in

Validation Steps

some cases

• when ai.enabled is true, the status icon should not be present
• regardless of chat.disableAIFeatures, the status icon should be present
• see above screenshots for how the status bar popup should look with different settings configurations

What to test

@:assistant

github auth behaviour

• signing into git source control or github pull requests in positron does not sign you into github for Assistant or Copilot Completions
• copilot status icon indicates as such (shows signed out)
• copilot models are not available in posit assistant chat
• if you already logged into github for something else in positron, then use the provider modal to log into github, you'll get a consent check "can Authentication use your github account"
• if you haven't logged into github at all, and you use the provider modal to log into github, you'll do the usual browser login flow
• logging into github in the provider modal enables github completions and next edit suggestions, the github status bar icon shows enabled, and copilot models are available with posit assistant

github provider enablement behaviour

• setting positron.assistant.provider.githubCopilot.enable to false and reloading positron disables copilot completions, makes the copilot status icon indicate disabled, and removes the github provider from the provider modal
• setting ai.enabled to false disable copilot completions and copilot next edit suggestions (along with all the other ai functionality in positron)

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:assistant

Why these tags?

Tag	Source
@:critical	Always runs (required)
@:assistant	PR description

More on automatic tags from changed files.

^{readme  valid tags}

[midleman]

@sharon-wang - can you add a proper mapping (or [] if there isn't one) for the new src dir as listed in the E2E test comment on this PR? 🙏

[sharon-wang]

@midleman yes thank you, just pushed 27aae9f to add the mapping!

[timtmok]

LGTM