Implement Enhanced Rate Limiting and Token Validation

[sujalsalekar]

This PR implements critical security and performance improvements to the rate limiting, token validation, and cache management systems.

Key Changes:

1. Token Validation Enhancements

   • Added strict Bearer token format validation
   • Implemented base64url encoding checks
   • Added minimum token length requirement (32 chars)
   • Improved security logging for invalid tokens

2. Rate Limiting Improvements

   • Implemented configurable per-token rate limits
   • Added concurrent request tracking
   • Differentiated limits for anonymous vs authenticated requests
   • Added proper cleanup of concurrent request counters
   • Fixed type safety issues with rate limit responses

3. Cache Management Optimization

   • Implemented Stale-While-Revalidate (SWR) pattern
   • Added configurable cache TTLs:
     • Default TTL: 1 hour
     • Stale window: 5 minutes
     • Background refresh: 55 minutes
   • Added background refresh for near-expiry cache entries
   • Improved error handling for cache operations

Technical Details:

• Added proper TypeScript type definitions
• Fixed KV store value handling (string-based storage)
• Implemented proper execution context for background tasks
• Added comprehensive error logging
• Fixed type safety issues with rate limit enum usage