Handle OAuth setup callback redirect from GitHub Apps.
This endpoint handles the “Setup URL” callback from GitHub Apps, which is triggered when a user installs or updates their GitHub App installation.
For “update” actions (user modified repo access via GitHub), we just show a success page since no token exchange is needed.
For new installations with code/state, we process similar to the regular OAuth callback.
Authorization code from OAuth provider
State parameter containing auth_id
Setup action from GitHub App
GitHub App installation ID
Error code from OAuth provider
Error description from OAuth provider
Successful Response