Use provider-aware OAuth handling instead of a single hardcoded client ID.
GitHub now follows VS Code's GitHub auth setup with the GitHub client ID,
GitHub authorize/token endpoints, prompt=select_account, vscode.dev/redirect,
and JSON-compatible token exchange. Entra keeps the Copilot client ID via a
known provider match, while unknown providers still fall back to metadata
and dynamic registration.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
