Fix: Disable RuboCop VerifiedDoubles check for Pro config mock

Added inline RuboCop disable comment for the Pro configuration double
since we're mocking a module from the Pro gem that doesn't exist in the
open-source test environment. Using a regular double is appropriate here.

Changes:
- Reverted complex instance_double setup
- Added `# rubocop:disable RSpec/VerifiedDoubles` inline comment
- RuboCop now passes with 0 offenses

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

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

Author: AbanoubGhadban

spec/react_on_rails/utils_spec.rb

@@ -248,7 +248,7 @@ def mock_dev_server_running
                 # Mock Pro gem being available
                 allow(described_class).to receive(:react_on_rails_pro?).and_return(true)
                 stub_const("ReactOnRailsPro", Module.new)
-                pro_config = double("ProConfiguration")
+                pro_config = double("ProConfiguration") # rubocop:disable RSpec/VerifiedDoubles
                 allow(ReactOnRailsPro).to receive(:configuration).and_return(pro_config)
                 allow(pro_config).to receive_messages(rsc_bundle_js_file: rsc_bundle_name,
                                                       react_server_client_manifest_file: nil)