Add response stub to mock request object

The HTTPX::ErrorResponse constructor calls request.response during
initialization. Add this method stub to the instance_double to prevent
the test from failing with:

  received unexpected message :response with (no args)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails_pro/spec/react_on_rails_pro/request_spec.rb

@@ -247,7 +247,7 @@
               yielder.call("#{original_chunks[0]}\n")
               # Simulate connection error mid-stream by creating a mock error response
               # Create a minimal mock request object that satisfies the ErrorResponse constructor
-              mock_request = instance_double(HTTPX::Request, uri: URI(render_full_url))
+              mock_request = instance_double(HTTPX::Request, uri: URI(render_full_url), response: nil)
               error_response = HTTPX::ErrorResponse.new(
                 mock_request,
                 StandardError.new("Connection closed")