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/codex-mcp/src/plugin_config.rs

@@ -257,6 +257,12 @@ fn normalize_plugin_mcp_server_value(
     }
 
     if let Some(JsonValue::Object(mut oauth)) = object.remove("oauth") {
+        if let Some(callback_url) = oauth.remove("callbackUrl") {
+            oauth
+                .entry("callback_url".to_string())
+                .or_insert(callback_url);
+        }
+
         if let Some(callback_port) = oauth.remove("callbackPort") {
             oauth
                 .entry("callback_port".to_string())