Add closed? method stub to streaming test mocks

Fixes 9 failing tests in react_on_rails_pro_helper_spec.rb by adding
the closed? method stub to mocked streams. The production code now
checks response.stream.closed? for client disconnect detection, so
test mocks need to support this method.

Changes:
- Add closed? stub returning false to stream_view_containing_react_components tests
- Add closed? stub returning false to cached_stream_react_component tests

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

Co-authored-by: Abanoub Ghadban <AbanoubGhadban@users.noreply.github.com>

Author: github-actions[bot]

react_on_rails_pro/spec/dummy/spec/helpers/react_on_rails_pro_helper_spec.rb

@@ -443,6 +443,7 @@ def mock_request_and_response(mock_chunks = chunks, count: 1)
           written_chunks << chunk
         end
         allow(mocked_stream).to receive(:close)
+        allow(mocked_stream).to receive(:closed?).and_return(false)
         mocked_response = instance_double(ActionDispatch::Response)
         allow(mocked_response).to receive(:stream).and_return(mocked_stream)
         allow(self).to receive(:response).and_return(mocked_response)
@@ -532,6 +533,7 @@ def execute_stream_view_containing_react_components
         written_chunks.clear
         allow(mocked_stream).to receive(:write) { |chunk| written_chunks << chunk }
         allow(mocked_stream).to receive(:close)
+        allow(mocked_stream).to receive(:closed?).and_return(false)
         mocked_response = instance_double(ActionDispatch::Response)
         allow(mocked_response).to receive(:stream).and_return(mocked_stream)
         allow(self).to receive(:response).and_return(mocked_response)