## Why
MCP servers can share an OAuth callback URL. Without a validated issuer or a
server-specific callback path, an authorization response could be associated
with the wrong server.
## What changed
- Use stable callbacks when authorization metadata advertises issuer-bound
responses, and validate the returned issuer before exchanging the code.
- Retain server-specific callback IDs for providers without issuer support,
including fallback to the global or default callback for legacy registered
clients.
- Persist registered callback URLs for MCP servers and plugins, and insert the
active listener port into portless loopback redirects.
## Testing
Add coverage for issuer validation, callback-mode discovery, registered and
legacy clients, plugin OAuth, CLI persistence, and loopback listener ports.
GitOrigin-RevId: 2878c92e237fc17fd3def0bd2e1cce3e104a3db8
9be8d6e1c3