Fix double error emission when htmlStream errors after startRSC

When htmlStream errors after startRSC is running, both the
htmlStream.on('error') handler and finished(htmlStream) rejection in
startRSC's catch block would emit on resultStream, causing duplicate
error reports. Now the htmlStream error handler only emits when startRSC
hasn't been called yet; once running, finished() handles it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AbanoubGhadban

packages/react-on-rails-pro/src/injectRSCPayload.ts

@@ -294,9 +294,16 @@ export default function injectRSCPayload(
    * propagate to handleStreamError → errorReporter in the node renderer.
    * Error alone is not the end of the stream — termination is handled by the
    * 'close' event below.
+   *
+   * We only emit here when startRSC hasn't been called yet. Once startRSC is
+   * running, finished(htmlStream) inside startRSC rejects on error, and the
+   * catch block there handles reporting — emitting here too would cause the
+   * same error to be reported twice.
    */
   htmlStream.on('error', (err) => {
-    resultStream.emit('error', err instanceof Error ? err : new Error(String(err)));
+    if (!rscPromise) {
+      resultStream.emit('error', err instanceof Error ? err : new Error(String(err)));
+    }
   });
 
   /**