The 2xx-but-missing-token/url branch echoed the parsed upstream response body
into the user-facing error. Return a generic message and log response_data
server-side only, mirroring the existing HTTPStatusError fix (Gap 4).
get_github_token_for_user folded the raw upstream response text into the
error string that becomes a Slack/Linear comment (AUTH-RESP-LEAK-01). Log the
full body server-side only and return a generic "GitHub auth failed (status
<code>)". Also document the accepted shared-installation-token blast radius on
the bot-token-only path (AUTHZ-003).
