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/tests/suite/mcp_startup_refresh_http_proxy.rs

@@ -416,6 +416,7 @@ async fn skill_mcp_dependency_oauth_uses_configured_http_client() -> Result<()>
             .and_then(|server| server.oauth.as_ref()),
         Some(&McpServerOAuthConfig {
             client_id: None,
+            callback_url: None,
             callback_port: Some(skill_callback_port),
         })
     );