Adopt the proposed authIssuers API for GitHub Enterprise

[TylerLeonhardt]

Feature request

Adopt the proposed authIssuers API introduced by microsoft/vscode#337846. GitHub Enterprise request destinations should come from the selected AuthenticationSession.authorizationServer, paired with that session's token, rather than from github-enterprise.uri.

• Remove the OAuth /login/oauth suffix and preserve existing GHES/GHE.com REST and GraphQL endpoint mapping, including deployment paths and ports.
• Use session provenance for repository matching, client reuse, host-specific caches, and scope upgrades.
• Keep enterprise enrollment/setup UI and public GitHub/PAT behavior. Report enterprise sessions without usable provenance as unavailable instead of falling back to the configured host.

This is consumer adoption only, not multiple-host configuration or a general authentication redesign. Production proposed-API allowlisting is handled separately in microsoft/vscode-distro.

Implementation: #8984.

[TylerLeonhardt]

Verification steps:

1. Use a VS Code 1.140+ build containing authentication: Add GitHub session issuer provenance vscode#337846 and the GitHub Pull Requests extension built from authentication: Use session issuers for enterprise routing #8984, with proposed APIs enabled.
2. Clone a GitHub Enterprise repository (create one here) with an open pull request, then open its folder in VS Code.
3. Open the GitHub Pull Requests view and sign in to GitHub Enterprise, entering your enterprise URL if prompted.
4. Refresh the view and open a pull request. Confirm its details and changed files load without authentication or API errors.