Polish configuration error message and add test comment (#1976)

## Summary

Two small improvements: better error message wording and a clarifying
test comment.

## Changes

- **`lib/react_on_rails/configuration.rb`**: Improve error message
wording
  - Remove extra comma and space
  - Remove "which may cause performance issues" phrase  
  - Reorder to show preferred `:defer` before `:sync`
  
- **`spec/dummy/spec/helpers/react_on_rails_helper_spec.rb`**: Add
comment explaining why defer is the default loading strategy

## Test Plan

- ✅ RuboCop passes
- ✅ No functional changes

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

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Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

lib/react_on_rails/configuration.rb

@@ -172,8 +172,8 @@ def validate_generated_component_packs_loading_strategy
 
       msg = <<~MSG
         ReactOnRails: Your current version of shakapacker \
-        does not support async script loading,  which may cause performance issues. Please either:
-        1. Use :sync or :defer loading strategy instead of :async
+        does not support async script loading. Please either:
+        1. Use :defer or :sync loading strategy instead of :async
         2. Upgrade to Shakapacker v8.2.0 or above to enable async script loading
       MSG
       if generated_component_packs_loading_strategy.nil?

spec/dummy/spec/helpers/react_on_rails_helper_spec.rb

@@ -77,6 +77,8 @@ def self.pro_attribution_comment
       allow(helper).to receive(:append_stylesheet_pack_tag)
       expect { helper.load_pack_for_generated_component("component_name", render_options) }.not_to raise_error
 
+      # Default loading strategy is now always :defer to prevent race conditions
+      # between component registration and hydration, regardless of async support
       expect(helper).to have_received(:append_javascript_pack_tag).with("generated/component_name", { defer: true })
       expect(helper).to have_received(:append_stylesheet_pack_tag).with("generated/component_name")
     end