Add direct unit tests for HTTPChunkedTransferDecoder

[ianegordon]

Summary

Existing chunked-body coverage runs through HTTPDecoder integration tests. This adds four tests that exercise HTTPChunkedTransferDecoder directly, pinning behavior the integration path doesn't cover:

• No over-consumption past the terminator — bytes after the chunked stream remain readable on the underlying source (the keep-alive / pipelining correctness property).
• nextBuffer(suggested:) honors the suggested cap — a 100-byte chunk requested with suggested: 16 yields only buffers ≤ 16 bytes.
• Uppercase HEXDIG accepted — FF\r\n… decodes 255 bytes (per RFC 5234 §2.3, ABNF literal strings are case-insensitive).
• Chunk-size exceeding Int.max rejected — a 20-hex-char value throws HTTPDecoder.Error rather than truncating or trapping. Distinct error path from non-hex input (already covered by invalidChunkSize_ThrowsError).

No public-API changes.

Test plan

• swift test --filter HTTPChunkedDecodedSequenceTests passes locally (4/4)
• CI green

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.75%. Comparing base (5295843) to head (1d1f0a9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #211   +/-   ##
=======================================
  Coverage   92.75%   92.75%           
=======================================
  Files          70       70           
  Lines        3645     3645           
=======================================
  Hits         3381     3381           
  Misses        264      264           

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