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-plugins/src/manager_tests.rs

@@ -982,6 +982,7 @@ async fn load_plugins_loads_default_skills_and_mcp_servers() {
                     scopes: None,
                     oauth: Some(McpServerOAuthConfig {
                         client_id: Some("client-id".to_string()),
+                        callback_url: None,
                         callback_port: Some(3118),
                     }),
                     oauth_resource: None,