feat: implement client_max_body_size validation for upload limits

[jos-felipe]

Summary

• Implements proper validation for the client_max_body_size configuration directive (10MB limit)
• Adds early request rejection for oversized uploads with HTTP 413 Payload Too Large responses
• Supports validation for both regular POST requests and chunked transfer encoding

Changes Made

• HttpRequest.hpp/cpp: Added server config reference and size validation methods
• Server.cpp: Updated to pass server configuration to request parsing
• test_upload_limits.sh: Comprehensive test suite covering all upload scenarios

Validation Points

1. Early Header Validation: Rejects requests with Content-Length > limit immediately
2. Incremental Body Validation: Checks accumulated body size during parsing
3. Chunked Transfer Validation: Validates chunk-by-chunk accumulation
4. Error Response: Returns proper 413 status with error page

Test Coverage

• ✅ Small file uploads (success cases)
• ✅ Large file uploads (rejection cases)
• ✅ Boundary file uploads (exactly at limit)
• ✅ Chunked transfer encoding
• ✅ Content-Length header validation
• ✅ Multipart form-data uploads

All tests pass and validate proper HTTP/1.1 compliance for request size limits.