feat(examples): enhance everything-server.yaml with comprehensive Lua validation scripts

- Add 7 sophisticated Lua validation scripts for advanced response analysis
- Mathematical precision validator: floating-point arithmetic with tolerance checking
- Large text analyzer: performance grading and content validation
- Unicode validator: multi-language and emoji character analysis
- Base64 image validator: PNG format verification and size estimation
- LLM response analyzer: content quality metrics and prompt responsiveness
- Progress performance validator: timing accuracy and performance grading
- Security environment validator: sensitive data detection and compliance

Enhanced test coverage:
- 15+ test cases now include custom Lua validation
- Rich context API access (request, response, metadata, session)
- Security sandbox with resource limits and monitoring
- Performance analysis with comprehensive metrics
- Enterprise-grade validation beyond basic JSONPath checking

Technical improvements:
- Validation scripts integrated via validation_scripts field in test cases
- Structured result format with success, message, and custom metrics
- Support for execution phases (before, after, both)
- Required/optional script designation for critical vs informational validation

This transforms the everything-server.yaml from basic functional testing to
enterprise-grade validation with sophisticated analysis capabilities.

Related to #196, #197, #198 - MCP server test specifications

Author: milliondreams