Add comprehensive test coverage and refactor extension

[johnbillion]

Summary

• ✅ Added comprehensive test infrastructure using @vscode/test-cli and mocha
• ✅ Created 17 tests covering all major functionality
• ✅ Refactored monolithic 618-line extension.ts into clean modular structure (now 14 lines!)
• ✅ Updated CI/CD workflow to run tests
• ✅ Updated documentation with test instructions

Test Coverage

The test suite includes:

• Hook completion (actions and filters)
• Callback completion (closures, arrow functions, utility functions)
• Hover provider with documentation links
• Edge cases (all hook functions, quotes, whitespace handling)

Breaking Changes

None - this is purely internal refactoring with comprehensive test coverage to ensure functionality remains unchanged.