Consolidate header field-pair validation into fields::is_field_valid

[yhirose]

Summary

The is_field_name(name) && is_field_value(value) predicate — the guard against CR/LF injection / response splitting — had accumulated in five separate output paths:

• set_header
• write_headers (server response headers, added in skip invalid fields in write_headers to prevent response splitting #2505)
• write_content_chunked trailer
• perform_websocket_handshake
• check_and_write_headers

The most recent addition (#2505) was a near-verbatim copy of the chunked-trailer skip block. This consolidates the shared predicate into a single fields::is_field_valid(name, value) helper and routes all five sites through it.

Notes

• No behavior change. Each site keeps its own follow-up action (emplace / continue / return false / set Error::InvalidHeaders); only the duplicated predicate is centralized.
• Follows the existing header-only split convention: forward declaration in the fields declaration block, inline definition in the implementation area next to is_field_value.
• Net −5 lines.

Testing

• make test_split builds clean.
• Response-splitting, chunked-trailer, WebSocket handshake, header-smuggling, and set_header tests all pass.