Remove Copilot SDK and use Copilot Chat for completions

[jmcphers]

This change eliminates Positron's dependence on the Copilot Language Server, and serves completions directly from an updated version of the GitHub Copilot Chat extension.

This solves a number of issues around authentication and feature parity with Copilot completions in VS Code.

Changes are also required to our Copilot Chat fork; see posit-dev/positron-copilot-chat#8.

Addresses #10491

Fixed: Auth state

Auth state is now derived from the built-in Accounts menu, instead of from the token that the LSP writes. This improves auth in remote SSH in particular, since the local authentication session can supply credentials to the chat and completion systems running on the the remote host.

#9811 - show correct state when signing out of copilot
#10365 - remote ssh auth again
#10317 - handle auth for remote ssh sessions
#8409 - also an issue for remote ssh auth

Fixed: Completion behavior

These issues are side effects of using the LSP, and should now be resolved.

#9301 - copilot completions emit a lot of console errors
#10564 - "trigger inline suggestion" command not supported
#11066 - copilot autocomplete does not work

Fixed: Working with a proxy

Now that all network requests are being made by the Copilot extension directly, VS Code's native network settings can be used to configure proxies and custom certificates.

https://docs.github.com/en/copilot/how-tos/configure-personal-settings/configure-network-settings

#8990 - certificate errors with sdk
#11015 - can't sign in to copilot
#8623 - issues signing in with a proxy

Partially fixed

#8775 - too many github sign ins. partially addressed; now copilot chat and completions share a sign in

Release Notes

New Features

• Use native implementation of Copilot completion engine, addressing a variety of authentication and performance issues. (Assistant: Remove Copilot LSP and use completions from Copilot extension #10491)
• Use local Copilot credentials in Remote SSH (Improve authentication for Copilot completions in remote SSH #8409)
• Use VS Code proxy settings when connecting to Github Copilot (Assistant: Issue signing into CoPilot with outgoing proxy #8623)

Bug Fixes

QA Notes

• You can now sign in or out of Copilot using the Accounts menu in addition to the Positron model config dialog.
• The Positron model config dialog will let you sign in but not sign out (a limitation of the authentication API). You'll need to sign out from the Accounts menu.
• An unfortunate side effect of this change is that we are once again using the Code OSS client ID when authenticating, which means that when you perform the auth flow the Github confirmation page will say "Visual Studio Code" of "Positron".

Test tags: @:assistant

[github-actions]

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

^{readme  valid tags}

[timtmok]

Lots of changes here so I'm going through some testing locally to see how authentication feels to the user.