Limit reassembled stream message size

[wasphin]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:

Each stream data frame is bounded by max_body_size, but continuation frames
can be accumulated into a message larger than that limit.

What is changed and the side effects?

Changed:

• Limit the accumulated payload of a fragmented stream message to
  max_body_size.
• Release pending fragments and close the stream with EMSGSIZE when the
  accumulated payload exceeds the limit.
• Cover the boundary and rejection behavior through a complete client/server
  RPC stream.

Side effects:

• Performance effects: one constant-time size check per stream data frame.
• Breaking backward compatibility: fragmented stream messages larger than
  max_body_size are now rejected instead of being accumulated without a
  bound.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR hardens bRPC streaming RPC framing by enforcing max_body_size on reassembled (fragmented) stream messages, preventing unbounded accumulation across continuation frames and rejecting oversized payloads with EMSGSIZE.

Changes:

• Enforce an accumulated-size cap for fragmented stream DATA frames in Stream::OnReceived, closing the stream with EMSGSIZE when the reassembled message would exceed max_body_size.
• Add an end-to-end client/server unit test that validates (1) acceptance exactly at the limit and (2) rejection just over the limit, using stream segmentation to force fragmentation.

Reviewed changes

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

File	Description
src/brpc/stream.cpp	Adds a per-DATA-frame constant-time check that prevents _pending_buf from growing beyond FLAGS_max_body_size, and closes the stream with EMSGSIZE while releasing pending fragments.
test/brpc_streaming_rpc_unittest.cpp	Adds an integration-style test covering the boundary (64 bytes) and over-limit (65 bytes) behaviors under forced fragmentation via stream_write_max_segment_size.

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[wwbmmm]

LGTM

[chenBright]

LGTM