Adopt the proposed authIssuers API for GitHub Enterprise #8985

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.