Improve shakapacker detection with robust multi-strategy approach

Enhanced shakapacker_in_gemfile? method to use layered detection:
1. Check Gem.loaded_specs (fastest)
2. Parse Gemfile.lock (most accurate, handles git/path gems)
3. Query Bundler.load.specs (reliable when available)
4. Fallback to Gemfile parsing

This approach is more efficient, avoids side effects from require(),
and handles edge cases better than the previous implementation.

Co-authored-by: User <[email protected]>
🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -182,20 +182,37 @@ def ensure_shakapacker_installed
         File.write(".shakapacker_just_installed", "")
       end
 
+      # Checks whether "shakapacker" is present in the *current bundle*,
+      # without loading it. Prioritizes Gemfile.lock (cheap + accurate),
+      # then Bundler's resolved specs, and finally a light Gemfile scan.
       def shakapacker_in_gemfile?
-        return false unless File.exist?("Gemfile")
+        gem_name  = "shakapacker"
+        gemfile   = ENV["BUNDLE_GEMFILE"] || "Gemfile"
+        lockfile  = File.join(File.dirname(gemfile), "Gemfile.lock")
 
-        gemfile_content = File.read("Gemfile")
-        return true if gemfile_content.match?(/gem\s+['"]shakapacker['"]/)
+        # 1) Already loaded in this process?
+        return true if Gem.loaded_specs.key?(gem_name)
 
-        # Also check if shakapacker is already available in the current bundle
-        # This handles cases where it might be loaded from parent environments
+        # 2) Present in the lockfile? (handles git/path gems too)
+        if File.file?(lockfile)
+          # Lines look like: "    shakapacker (x.y.z)"
+          return true if File.foreach(lockfile).any? { |l| l.match?(/^\s{4}#{Regexp.escape(gem_name)}\s\(/) }
+        end
+
+        # 3) Ask Bundler for resolved specs (doesn't require the gem code)
         begin
-          require "shakapacker"
-          true
-        rescue LoadError
-          false
+          require "bundler"
+          return true if Bundler.load.specs.any? { |s| s.name == gem_name }
+        rescue StandardError
+          # Fall through if Bundler isn't available or load fails
+        end
+
+        # 4) Fallback: direct Gemfile mention (not perfect, but cheap)
+        if File.file?(gemfile)
+          return true if File.foreach(gemfile).any? { |l| l.match?(/^\s*gem\s+['"]#{Regexp.escape(gem_name)}['"]/) }
         end
+
+        false
       end
 
       def add_bin_scripts