feat: add security utilities and input validation

[CrazyDubya]

Security Enhancements: Input Validation and Sanitization

Added comprehensive security utilities and custom exception hierarchy:

• Custom exceptions (TinyTroupeException, SecurityException, ValidationException, LLMException, MemoryException)
• Input sanitization to prevent XSS and injection attacks
• Prompt/response length validation
• JSON structure validation

Security Protections:

• Blocks script tags, javascript protocol, event handlers, iframes
• Input length limits and truncation
• Type checking and required field validation
• Token-based response length limits

Testing:

• 15 comprehensive unit tests covering all validation functions
• Verified XSS prevention, input sanitization, and length validation

Files Added: 3 files (+218 lines, -106 lines)
Impact: Significantly improved security posture for production use

This extracts security improvements from the closed PR #153 as a focused, reviewable enhancement.

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