Fix critical missing StringIO require and test stubbing issues

This commit addresses critical issues identified in code review:

1. **Add missing StringIO require** (BLOCKER)
   - Added `require "stringio"` at line 4
   - Required for capture_output method at line 91
   - Without this, code fails at runtime in silent mode with:
     NameError: uninitialized constant StringIO

2. **Fix inconsistent system call stubbing in tests**
   - Updated "when not in Bundler context" tests to match actual implementation
   - Changed from string form: `system("bundle exec rake...")`
   - To array form: `system("bundle", "exec", "rake", ...)`
   - Silent mode now correctly stubs with File::NULL options
   - This matches the actual implementation at lines 116-121

3. **Fix Rails fallback test stubbing**
   - Updated "when Rails is not available" test
   - Now uses correct array form system call signature

All tests pass (10 examples, 0 failures)
RuboCop passes with no offenses

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

lib/react_on_rails/dev/pack_generator.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "English"
+require "stringio"
 
 module ReactOnRails
   module Dev

spec/react_on_rails/dev/pack_generator_spec.rb

@@ -110,28 +110,36 @@
       end
 
       it "runs pack generation successfully in verbose mode using bundle exec" do
-        command = "bundle exec rake react_on_rails:generate_packs"
-        allow(described_class).to receive(:system).with(command).and_return(true)
+        allow(described_class).to receive(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs")
+          .and_return(true)
 
         expect { described_class.generate(verbose: true) }
           .to output(/📦 Generating React on Rails packs.../).to_stdout_from_any_process
 
-        expect(described_class).to have_received(:system).with(command)
+        expect(described_class).to have_received(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs")
       end
 
       it "runs pack generation successfully in quiet mode using bundle exec" do
-        command = "bundle exec rake react_on_rails:generate_packs > /dev/null 2>&1"
-        allow(described_class).to receive(:system).with(command).and_return(true)
+        allow(described_class).to receive(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs",
+                out: File::NULL, err: File::NULL)
+          .and_return(true)
 
         expect { described_class.generate(verbose: false) }
           .to output(/📦 Generating packs\.\.\. ✅/).to_stdout_from_any_process
 
-        expect(described_class).to have_received(:system).with(command)
+        expect(described_class).to have_received(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs",
+                out: File::NULL, err: File::NULL)
       end
 
       it "exits with error when pack generation fails" do
-        command = "bundle exec rake react_on_rails:generate_packs > /dev/null 2>&1"
-        allow(described_class).to receive(:system).with(command).and_return(false)
+        allow(described_class).to receive(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs",
+                out: File::NULL, err: File::NULL)
+          .and_return(false)
 
         expect { described_class.generate(verbose: false) }.to raise_error(SystemExit)
       end
@@ -148,13 +156,15 @@
       end
 
       it "falls back to bundle exec when Rails is not defined" do
-        command = "bundle exec rake react_on_rails:generate_packs"
-        allow(described_class).to receive(:system).with(command).and_return(true)
+        allow(described_class).to receive(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs")
+          .and_return(true)
 
         expect { described_class.generate(verbose: true) }
           .to output(/📦 Generating React on Rails packs.../).to_stdout_from_any_process
 
-        expect(described_class).to have_received(:system).with(command)
+        expect(described_class).to have_received(:system)
+          .with("bundle", "exec", "rake", "react_on_rails:generate_packs")
       end
     end
   end