Improve pack generation to avoid unnecessary subprocess spawning

Refactors pack_generator.rb to intelligently choose between direct Rake
task execution and bundle exec based on the current context. When running
in a Rails/Bundler environment (e.g., from bin/dev), the pack generation
now runs the Rake task directly, avoiding the overhead of spawning a
subprocess.

Key improvements:
- Adds run_pack_generation method that detects Rails availability
- Implements run_rake_task_directly for in-process execution when Rails is available
- Falls back to run_via_bundle_exec when Rails is not available
- Includes proper silent mode handling for both execution paths
- Prevents dead code by ensuring Rails is always defined before use

This change is based on the optimization from the shakapacker-9.3.0 branch
(commit b716f08), which already excluded the dead code that would have
checked for Rails being defined when it's guaranteed to be present.

Fixes #1909

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

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

Author: justin808

lib/react_on_rails/dev/pack_generator.rb

@@ -9,10 +9,10 @@ class << self
         def generate(verbose: false)
           if verbose
             puts "📦 Generating React on Rails packs..."
-            success = system "bundle exec rake react_on_rails:generate_packs"
+            success = run_pack_generation
           else
             print "📦 Generating packs... "
-            success = system "bundle exec rake react_on_rails:generate_packs > /dev/null 2>&1"
+            success = run_pack_generation(silent: true)
             puts success ? "✅" : "❌"
           end
 
@@ -21,6 +21,63 @@ def generate(verbose: false)
           puts "❌ Pack generation failed"
           exit 1
         end
+
+        private
+
+        def run_pack_generation(silent: false)
+          # If we're already inside a Bundler context AND Rails is available (e.g., called from bin/dev),
+          # we can directly require and run the task. Otherwise, use bundle exec.
+          if defined?(Bundler) && rails_available?
+            run_rake_task_directly(silent: silent)
+          else
+            run_via_bundle_exec(silent: silent)
+          end
+        end
+
+        def rails_available?
+          return false unless defined?(Rails)
+          return false unless Rails.respond_to?(:application)
+          return false if Rails.application.nil?
+
+          true
+        end
+
+        def run_rake_task_directly(silent: false)
+          require "rake"
+
+          # Load tasks only if not already loaded (don't clear all tasks)
+          Rails.application.load_tasks unless Rake::Task.task_defined?("react_on_rails:generate_packs")
+
+          if silent
+            original_stdout = $stdout
+            original_stderr = $stderr
+            $stdout = StringIO.new
+            $stderr = StringIO.new
+          end
+
+          begin
+            task = Rake::Task["react_on_rails:generate_packs"]
+            task.reenable # Allow re-execution if called multiple times
+            task.invoke
+            true
+          rescue StandardError => e
+            warn "Error generating packs: #{e.message}" unless silent
+            false
+          ensure
+            if silent
+              $stdout = original_stdout
+              $stderr = original_stderr
+            end
+          end
+        end
+
+        def run_via_bundle_exec(silent: false)
+          if silent
+            system "bundle exec rake react_on_rails:generate_packs > /dev/null 2>&1"
+          else
+            system "bundle exec rake react_on_rails:generate_packs"
+          end
+        end
       end
     end
   end