Decrement remainingStreamingBytes before dataHandler calls; simplify isLast check

[Copilot]

maxRemainingBodyLength() inside onData callbacks returned the byte count including the current chunk, breaking the invariant isLast == (maxRemainingBodyLength() == 0) for both content-length and chunked transfers.

Changes

• Content-length: Decrement remainingStreamingBytes before calling dataHandler (was after), then pass remainingStreamingBytes == 0 directly as isLast — no local variable needed.

  // Before
  dataHandler(user, std::string_view(data, emittable), emittable == remainingStreamingBytes);
  remainingStreamingBytes -= emittable;
  
  // After
  remainingStreamingBytes -= emittable;
  dataHandler(user, std::string_view(data, emittable), remainingStreamingBytes == 0);

• Chunked encoding: For the terminal empty chunk, temporarily set remainingStreamingBytes = 0 before calling dataHandler, then restore the saved state so ChunkIterator can still drop the trailing CRLF on its next step.

Applied consistently across all three call sites: fenceAndConsumePostPadded, the direct path in consumePostPadded, and the fallback path in consumePostPadded.

---

📱 Kick off Copilot coding agent tasks wherever you are with GitHub Mobile, available on iOS and Android.