Clean solution: Remove redundant using_packer? check with proper test handling

Since Shakapacker is required by gemspec, the using_packer? check in
bundle_js_file_path is unnecessary for production code. However, tests
mock this scenario for validation.

Changes:
- Remove using_packer? check from main bundle_js_file_path logic
- Add guard check in bundle_js_file_path_with_packer for test scenarios
- Maintains clean production code while handling test mocking properly
- All tests pass including "without packer" scenarios

This is the correct approach: main logic assumes Shakapacker is available
(as guaranteed by gemspec), while method implementation handles edge cases
for comprehensive test coverage.

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

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

Author: justin808

lib/react_on_rails/utils.rb

@@ -72,7 +72,7 @@ def self.bundle_js_file_path(bundle_name)
       # Priority order depends on bundle type:
       # SERVER BUNDLES (normal case): Try secure non-public locations first, then manifest, then legacy
       # CLIENT BUNDLES (normal case): Try manifest first, then fallback locations
-      if bundle_name == "manifest.json" || !ReactOnRails::PackerUtils.using_packer?
+      if bundle_name == "manifest.json"
         # Default to the non-hashed name in the specified output directory, which, for legacy
         # React on Rails, this is the output directory picked up by the asset pipeline.
         # For Shakapacker, this is the public output path defined in the (shaka/web)packer.yml file.
@@ -83,6 +83,9 @@ def self.bundle_js_file_path(bundle_name)
     end
 
     def self.bundle_js_file_path_with_packer(bundle_name)
+      # Handle test scenarios where packer is mocked as unavailable
+      return File.join(generated_assets_full_path, bundle_name) unless ReactOnRails::PackerUtils.using_packer?
+
       is_server_bundle = server_bundle?(bundle_name)
 
       if is_server_bundle