[codex] Limit multipart header block size

[chrislearn]

Summary

• add an explicit size limit for each field's full header block
• reject oversized multipart headers with a dedicated HeadersSizeExceeded error
• add integration coverage for both accepted and rejected header sizes

Why

Multipart header parsing previously depended on the whole-stream limit for memory safety. If callers left whole_stream unbounded, a request with an oversized header block could grow memory until EOF or until the separator was found.

Impact

Callers now get a finite default header-block limit and can override it through SizeLimit::headers(...) when needed.

Validation

• cargo test --all --all-features
• cargo clippy --tests --examples --bins -- -D warnings

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