Reject malformed Host header ports

[bneradt]

Reject malformed Host field authorities before ATS reuses them as request targets or cached host state. Shared Host parsing now requires host or host:port syntax, enforces bracketed IPv6 when a port is present, and rejects invalid or out-of-range ports.

This patch adds http_parse_host_header, a new host parser function which also validates the required host header format. The patch also updates the various host header parsing logic blocks across ATS to use the new parser.

Reported-by: Ankit Singh

Fixes: #13049

[Copilot]

Pull request overview

This PR hardens ATS Host header handling to prevent malformed authorities (notably multiple-colon “ports”) from being reused as request targets or cached host state, addressing the reported URLImpl::set_port() overflow vector.

Changes:

• Added http_parse_host_header() to parse/validate Host field values (host or host:port, IPv6 bracket rules, strict numeric port range).
• Updated several call sites to rely on the new parser instead of ad-hoc : splitting / ink_atoi parsing.
• Added regression coverage via a new gold replay test and new Catch2 unit tests around Host parsing and promotion behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/gold_tests/headers/replays/malformed_host_port.replay.yaml	New replay intended to assert ATS rejects Host values with multiple port separators.
tests/gold_tests/headers/malformed_host_port.test.py	New gold test wrapper executing the replay.
src/proxy/http/remap/UrlRewrite.cc	Uses http_parse_host_header() when deriving host/port from Host for redirect mapping.
src/proxy/http/HttpTransact.cc	Uses http_parse_host_header() to safely populate pristine host/port into cache lookup URL.
src/proxy/http/HttpSM.cc	Uses http_parse_host_header() when extracting host/port for unmapped_url during remap.
src/proxy/hdrs/unit_tests/test_Hdrs.cc	Adds unit tests for http_parse_host_header() and Host validation/promotion behaviors.
src/proxy/hdrs/HTTP.cc	Implements http_parse_host_header() and rewires Host validation + Host cache fill logic to use it.
include/proxy/hdrs/HTTP.h	Declares http_parse_host_header() and documents Host validation behavior.