perf: reduce header validation and pseudo-header extraction passes

[anmonteiro]

Summary

• replace valid_headers's multi-pass checks with a single linear scan that validates case, pseudo-header ordering/legality, and connection-specific header constraints
• add a request-header scan helper that captures :method, :scheme, :path, and :authority while validating headers
• update method_path_and_scheme_or_malformed to use scan output instead of repeated get_multi_pseudo/mem traversals

Testing

• dune build --no-buffer @runtest-test_h2 @runtest-test_h2_client @runtest-test_h2_server @runtest-test_priority