Harden MCP OAuth callback handling (#40691)

## 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

codex-rs/core/config.schema.json

@@ -1920,6 +1920,10 @@
           "minimum": 0.0,
           "type": "integer"
         },
+        "callback_url": {
+          "description": "Registered callback URL associated with this OAuth client.",
+          "type": "string"
+        },
         "client_id": {
           "description": "Explicit OAuth client identifier to present during authorization and token exchange.",
           "type": "string"