Validate returned stream identifiers

[wasphin]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

When a client creates multiple streams in one RPC, it expects the server response to return exactly one extra_stream_id for every stream after the first. The response path indexed this list using the number of locally created streams without first validating its size. A malformed response could therefore read past the returned identifiers, while surplus identifiers had no valid client-stream mapping.

What is changed and the side effects?

Changed:

• Require the number of returned extra_stream_ids to exactly match the number of additional client streams.
• Fail the RPC with ERESPONSE and use the existing stream cleanup path when the counts differ.
• Add full client/server regression coverage for both missing and surplus returned stream identifiers.

Side effects:

• Performance effects: One integer comparison is added when completing a multi-stream RPC.

• Breaking backward compatibility: Malformed responses with missing or surplus stream identifiers are now rejected instead of being partially processed.

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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

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This PR hardens multi-stream RPC handling by validating that the server returns exactly one extra_stream_id per additional client stream, and adds regression coverage for both missing and surplus returned identifiers.

Changes:

• Add a client-side validation that extra_stream_ids_size == stream_num - 1 and fail with ERESPONSE on mismatch.
• Introduce a test service that intentionally returns mismatched extra_stream_ids.
• Add a unit test asserting the mismatch is rejected and that streams are cleaned up.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
test/brpc_streaming_rpc_unittest.cpp	Adds regression tests for missing/surplus extra_stream_ids using a custom test service.
src/brpc/controller.cpp	Adds validation of returned extra_stream_ids count and fails the RPC on mismatch.

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