read GOAWAY fields before Additional Debug Data in OnGoAway

[ubeddulla]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:

1. RFC 7540 section 6.8 puts Last-Stream-ID and Error Code at the front of the GOAWAY payload with the Additional Debug Data after them, but OnGoAway skips payload_size - 8 bytes first and then reads the trailing 8 bytes, so a peer that attaches debug data gets to choose the Last-Stream-ID we act on.
2. It also keeps the reserved high bit, so a 0xff debug byte makes last_stream_id negative, and _goaway_stream_id then never passes the >= 0 test in TryToInsertStream, leaving the client free to open new streams on a connection the peer has already abandoned.

What is changed and the side effects?

Changed: moved the two reads ahead of the forward and masked the reserved bit, the way ConsumeFrameHead and OnWindowUpdate already treat theirs. The new http2_goaway_with_debug_data case sends a GOAWAY announcing stream 1 with 8 bytes of debug data and checks _goaway_stream_id ends up as 1 instead of -1.

Side effects:

• Performance effects: none, same number of bytes consumed from the iterator.

• Breaking backward compatibility: no. A GOAWAY carrying no debug data parses exactly as before.

[Copilot]

Pull request overview

This PR fixes HTTP/2 GOAWAY frame parsing in brpc::policy::H2Context::OnGoAway to follow RFC 7540 layout and prevent peers from influencing Last-Stream-ID via Additional Debug Data. It also masks off the reserved high bit of Last-Stream-ID to avoid negative values affecting stream admission logic.

Changes:

• Read Last-Stream-ID and Error Code from the beginning of the GOAWAY payload (before skipping debug data).
• Mask off the reserved bit in Last-Stream-ID on receipt (& 0x7FFFFFFF).
• Add a unit test covering GOAWAY with Additional Debug Data and reserved-bit handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/brpc/policy/http2_rpc_protocol.cpp	Fix GOAWAY parsing order and reserved-bit handling to prevent malformed/malicious debug data from affecting Last-Stream-ID.
test/brpc_http_rpc_protocol_unittest.cpp	Add regression test for GOAWAY frames that include debug data and reserved-bit set in Last-Stream-ID.

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