Accept CONNECT and absolute-form request-targets (RFC 9112 §3.2)

[CodyPubNub]

Resolves #909. Alternative to #912.

Summary

• Accept absolute-form request targets such as GET http://host/path.
• Accept authority-form only for valid CONNECT host:port targets.
• Preserve scheme and authority on the parsed Uri, while forwarding
  absolute-form to an origin as origin-form.
• Keep RequestHeader unchanged when set_raw_path() rejects a replacement.
• Return / for non-CONNECT URIs without a path-and-query, and limit the
  authority fallback to CONNECT.

Why

RequestHeader::set_raw_path() previously sent every request target through
Uri::builder().path_and_query(). Starting with http 1.4.1, that builder
rejects targets other than origin-form or *. This makes valid HTTP/1
absolute-form and CONNECT authority-form requests fail before application code
runs.

RFC 9112 §3.2.2 requires servers to accept absolute-form. RFC 9112 §3.2.3 defines CONNECT authority-form as uri-host ":" port.

Behavior

Request target	Stored path	Stored authority	raw_path() / origin wire
/path?q	/path	—	/path?q
*	*	—	*
http://host/path?q	/path	host	/path?q
http://host	/	host	/
http://host?q	/	host	/?q
host:443 with CONNECT	""	host:443	host:443

Malformed CONNECT targets are rejected, including targets with a scheme, path,
query, userinfo, missing or invalid port, empty host, malformed IP-literal, or
invalid reg-name characters.

Implementation

• parse_request_target() computes the new Uri and raw fallback before
  updating either field, so errors leave the existing header intact.
• Absolute-form is parsed as a full Uri. Scheme and authority remain
  available to callers; the stored path-and-query is normalized for later H1
  or H2 forwarding.
• CONNECT targets retain their original bytes for authority-form
  serialization and are validated as uri-host ":" port.
• Bracketed hosts accept IPv6 and the RFC 3986 IPvFuture grammar. Unbracketed
  hosts accept the http::Uri-representable subset of reg-name.
• Origin-form and asterisk-form retain their existing path-and-query parsing.

No new fields are added. The common origin-form path does not perform
authority validation.

Interaction with current main

0c08149 independently fixed the same raw_path() panic by falling back to the Uri authority unconditionally. This branch retains that fallback for CONNECT and intentionally narrows it for other methods, which return / instead. Authority-form is valid only for CONNECT (§3.2.3); origin-form requires at least / (§3.2.1). The upstream test_authority_form_raw_path test is kept and passes unchanged.

Tests

• pingora-http unit tests cover all four request-target forms, malformed
  absolute and authority targets, IPv6, IPvFuture, reg-name, and atomic failure.
• pingora-core tests drive absolute-form and CONNECT through the real stock
  HTTP/1 downstream parser using mock IO, including a malformed
  CONNECT [gg]:443 that is rejected at read time — validation fires on actual
  inbound H1 traffic, not just on constructed headers.
• H1 request-line serialization is checked byte-for-byte for absolute-form and
  CONNECT.
• Existing allowed and disallowed H1 CONNECT proxy tests now run without the
  patched_http1 feature.
• A proxy integration test sends absolute-form inbound and verifies the origin
  receives GET /absolute?q=1 HTTP/1.1.

Local verification:

• cargo test -p pingora-http --lib: 52 passed
• cargo test -p pingora-core --lib request_target: 4 passed
• cargo test -p pingora-proxy --lib: 22 passed
• cargo test -p pingora-proxy --test test_basic --no-run: clean
• cargo check --workspace: clean
• cargo fmt --all -- --check: clean
• git diff --check: clean

The three CONNECT/absolute-form proxy integrations compile locally but require the external OpenResty fixture and have not run locally. Stock-parser behavior is covered locally by the pingora-core mock-IO tests; the integrations will execute in CI.

Difference from #912

• Absolute-form without a path is normalized to /, rather than re-emitting
  the absolute URI as the origin request target.
• http://host?q is normalized to /?q on the stored Uri, so a later H2
  :path rebuild is valid.
• CONNECT validation rejects shapes that http::Uri accepts but RFC 9112
  authority-form does not.

Out of scope

• Replacing Host from the absolute-form authority.
• Preserving absolute-form when forwarding to another proxy rather than an
  origin.
• Rewriting server-wide OPTIONS absolute-form to *.
• Non-UTF-8 targets without a leading slash.
• Percent-encoded reg-names, which http::Uri rejects before host validation.