- Add HEAD, OPTIONS, TRACE method handlers to complete_test_resource
- Add integration tests for HEAD, OPTIONS, TRACE HTTP methods
- Add request_info_resource and test for get_requestor(), get_requestor_port(), get_version()
- Add content_limit_suite to test content_too_large() with content_size_limit
- Add unregister_then_404 test for webserver::unregister_resource()
- Create http_response unit test file with tests for response code, headers, footers, and cookies

These tests increase coverage for previously untested code paths in
webserver.cpp, http_request.cpp, and http_response.cpp.

- Add IPv6 and dual-stack webserver tests
- Add HTTPS webserver and TLS session getter tests with graceful failure handling
- Add bind_address_ipv4 test with runtime URL building
- Add shoutcast response test for MHD_ICY_FLAG verification
- Add string_response constructor tests
- Remove CONNECT method test (incompatible with curl's tunneling semantics)

Coverage improved from ~90% to 91.8% lines, 93.3% functions.

Add --print-summary flag to gcovr command in CI to help diagnose
the discrepancy between local coverage (91.8%) and Codecov (64.53%).

The coverage report was including 15,781 lines from system headers and
dependencies instead of just the ~1,500 lines of libhttpserver source.
Use --filter to restrict coverage to src/ directory only.

- Add footer/trailer tests for http_request and http_response
- Add PSK handler configuration tests (setup, empty handler, no handler)
- Add ban/allow IP weight comparison tests for specific-then-wildcard case
- Add auth skip path tests for root path and wildcards
- Add IPv6 parsing edge case tests (too many parts, wildcards, nested)
- Add http_endpoint invalid regex pattern test
- Add custom error handler tests (not_found, method_not_allowed)
- Add request info caching tests

Branch coverage: 60.4% -> 64.2% (+65 branches)
Line coverage: 91.8% -> 94.9%

- Add null value query parameter test covering nullptr branches in
  build_request_args and build_request_querystring (lines 234, 248)
- Add digested user caching tests for cache hit and nullptr branches
  (lines 293-295, 300) in http_request.cpp
- Add caret prefix URL pattern tests covering line 85 in http_endpoint.cpp
- Add consecutive slashes URL test covering empty parts handling (line 83)

Branch coverage improvements:
- http_endpoint.cpp: 66.9% -> 68.7%
- http_request.cpp: 58.5% -> 59.7%

- Add tests for all HTTP methods falling through to base render()
- Add custom internal error resource handler test
- Add get_arg_flat fallback test
- Add large multipart form field test (100KB)
- Add file upload with explicit content-type header test
- Add http_endpoint tests for regex validation and error paths
- Add tests for invalid URL parameter formats

- Replace static global strings with function-static pattern
- Change 'long' to 'int64_t' for http_code variables
- Add missing #include <utility> for std::move

- Fix file_upload tests to use subdirectory instead of /tmp for
  cross-platform compatibility
- Extract hex validation functions (is_valid_hex, hex_char_to_val)
  from webserver.cpp to string_utilities for testability
- Add unit tests for hex utility functions to improve coverage

On Windows/MinGW, mkdir() only takes one argument (path), while
POSIX mkdir() takes two (path and mode). Add a MKDIR macro that
uses the appropriate signature for each platform.