feat(executor): implement script integration into TestCaseExecutor for Issue #256

- Add ScriptManager for managing validation scripts and validators
- Enhance TestCaseExecutor with script execution in before/after phases
- Implement ScriptValidationResult with comprehensive error handling
- Add script execution metrics to ExecutionMetrics structure
- Integrate script results into TestCaseResult for full workflow support
- Add comprehensive async tests covering all script integration scenarios
- Support both required and optional script validation modes
- Implement mock MCP client responses for GREEN phase testing
- Add simulated script execution with failure pattern detection
- Support conditional script logic and timeout/error scenarios

Design doc: docs/design/issue-256-testcaseexecutor-script-integration.md
Tests: 17 script integration tests, all passing
Coverage: All execution phases, error handling, and edge cases tested
Breaking changes: None - maintains backward compatibility

closes #256

Author: milliondreams