feat: add HTTP/1.1 request serializer

[zuudevs]

Summary

Adds the internal HTTP/1.1 request serializer on top of the merged Request, Headers, and Url data model.

Added

• src/http/request_serializer.hpp
  • SerializedRequest with parsed Url, owned header block, and borrowed body view
  • avoids copying request body bytes just to build wire data
• src/http/request_serializer.cpp
  • serializes all v1 methods: GET, HEAD, POST, PUT, PATCH, DELETE
  • emits origin-form request target and HTTP/1.1
  • automatically generates Host when absent
  • formats bracketed IPv6 literals correctly in generated Host
  • preserves explicit URL ports in Host
  • serializes caller headers in insertion order
  • automatically generates Content-Length for non-empty bodies when absent
  • validates caller-provided Content-Length against body size
  • rejects duplicate Host / Content-Length
  • rejects request Transfer-Encoding because v1 request chunking is out of scope
  • validates header names as HTTP token characters
  • rejects CR/LF/control-byte header injection
• tests/request_serializer_test.cpp
  • exact GET wire framing
  • all supported methods
  • duplicate custom headers
  • caller-provided Host override
  • explicit port and IPv6 Host formatting
  • zero-copy body view
  • Content-Length generation/validation
  • invalid header/framing cases
  • URL error propagation

Serialization model

The serializer intentionally produces two output segments: an owned request head and the original borrowed request body. Future Client execution can therefore call write_all(head) followed by write_all(body) without allocating a second full request buffer.

Scope

This PR does not write bytes to a socket, parse responses, implement redirects, or add Client. The next step can build the incremental HTTP/1.1 response parser on top of TcpConnection::read_some().

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject whitespace-only Host values

When the caller supplies a whitespace-only Host value such as "\t", valid_header_value accepts the tab and this raw emptiness check passes. Serialization then succeeds with a semantically empty Host header while suppressing generation from the URL, producing an invalid HTTP/1.1 request that servers can reject. Check for an empty value after removing optional whitespace, or fully validate the Host authority.

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