Fix HPACK dynamic table size update recursive decoding

[wwbmmm]

What problem does this PR solve?

Issue Number: null

Problem Summary:

HPACK decoding handled Dynamic Table Size Update entries by recursively calling
the decoder again. A header block containing many consecutive table size updates
could therefore use a large amount of call stack before reaching a real header
field or the end of input.

What is changed and the side effects?

Changed:

• Decode consecutive Dynamic Table Size Update entries iteratively.
• Preserve the total consumed byte count when table size updates precede a
  decoded header field.
• Add HPACK regression tests for long update sequences and update-before-header
  decoding.

Side effects:

• Performance effects: avoids recursive call overhead for consecutive table
  size updates.

• Breaking backward compatibility: none.

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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 fixes HPACK decoding behavior for Dynamic Table Size Update entries by removing recursive re-entry into the decoder, preventing deep call stacks when many consecutive updates appear, and ensuring the returned “bytes consumed” count includes any leading table size updates.

Changes:

• Replace recursive handling of consecutive Dynamic Table Size Update entries with an iterative decode loop.
• Preserve total consumed byte count when size updates precede a decoded header field.
• Add regression tests covering long update sequences and update-before-header decoding.

Reviewed changes

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

File	Description
src/brpc/details/hpack.cpp	Makes HPACK decode skip consecutive dynamic table size updates iteratively and correctly accumulate consumed bytes before returning a decoded header.
test/brpc_hpack_unittest.cpp	Adds regression tests to prevent stack-overflow regressions and to verify consumed-byte accounting when updates precede a header.

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[chenBright]

LGTM