Fix H2 origin payload handling

[bneradt]

This breaks out HTTP/2 work from this otherwise HTTP/3 focused PR:
#13213

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HTTP/2 origin responses can legally carry a non-zero Content-Length on
responses that do not carry a payload, such as responses to HEAD
requests. ATS was validating these responses without retaining the
request method that had been sent on the outbound H2 stream, so it could
mistake a valid no-body response for a payload length error.

Outbound H2 DATA generation also needs to respect the transaction write
VIO byte count when a buffer reader has more bytes available than should
be sent on the stream. This caps the frame payload length to the
remaining write VIO bytes and still marks END_STREAM when that payload
finishes the transaction body.

This extends the H2 origin replay coverage with a HEAD response that has
a non-zero Content-Length and no body, plus a large PUT request and
response over an H2 client and H2 origin path.

[Copilot]

Pull request overview

This PR fixes HTTP/2 origin-side edge cases in payload handling by preserving the outbound request method for response validation (e.g., HEAD with non-zero Content-Length but no body) and by capping outbound DATA frame payloads to the remaining write VIO byte count. It also extends the existing H2 origin replay gold tests to cover these regressions.

Changes:

• Track the sent outbound request method on an H2 stream and use it when deciding whether Content-Length vs. received payload-length mismatches are valid for no-body responses.
• Limit outbound H2 DATA frame payload length to the remaining write VIO bytes while still properly emitting END_STREAM when the transaction body is complete.
• Extend H2 origin replay coverage (HEAD no-body w/ non-zero Content-Length, large PUT + response) and update expected metric outputs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/proxy/http2/Http2ConnectionState.cc	Caps DATA frame payload by remaining write VIO bytes; records the outbound request method when sending request headers.
include/proxy/http2/Http2Stream.h	Adds write_vio_ntodo() accessor; uses stored sent request method for payload-length validation logic.
tests/gold_tests/h2/replay_h2origin/h2-origin.yaml	Adds new replay scenarios for HEAD (non-zero Content-Length, no body) and large PUT request/response.
tests/gold_tests/h2/h2origin.test.py	Updates squid log expectations for the new UUID ranges.
tests/gold_tests/h2/gold/http-request-method-metrics.gold	Updates the expected PUT request metric count to reflect the new PUT test.

Comments suppressed due to low confidence (1)

include/proxy/http2/Http2Stream.h:429

• Typo in the Warning message key: "data_legnth" should be "data_length" to make the log output searchable/consistent.

  if (content_length != 0 && !is_payload_precluded && content_length != data_length) {
    Warning("Bad payload length content_length=%d data_legnth=%d session_id=%" PRId64, content_length,
            static_cast<int>(data_length), _proxy_ssn->connection_id());

[bneradt]

[approve ci]