Don't set DATA send flag for zero-length send requests

[Tanishq Sharma (Tani-shar)]

Fixes #6243.

1. The change: skip QuicSendSetStreamSendFlag(..., QUIC_STREAM_SEND_FLAG_DATA, ...) in QuicStreamSendFlush when SendRequest->TotalLength == 0.

2. Why this option over the other two in the issue: clearing DATA when a flush finds nothing sendable would also fire when the peer hasn't allowed the stream or when flow control blocks it, QuicStreamSendCanWriteDataFrames treats those as separate branches from "no unsent data" , so that fix would wedge streams a different way. Rejecting zero-length sends outright changes public API behavior.

3. FIN is unaffected: QuicStreamSendShutdown sets QUIC_STREAM_SEND_FLAG_FIN directly on the graceful path, so a zero-length send with FIN still works.

4. Verified: repro from the issue reproduces on 4984c21 (2.7.0), PASS after the fix, SKIP_EMPTY_SEND=1 still PASS, and SEND_COMPLETE still fires for the zero-length request.

Update: added a regression test and a second guard.

1. Guard 1 (above) skips setting DATA for zero-length requests in
   QuicStreamSendFlush.

2. Guard 2 clears a stale DATA flag in the stream write path when the flag is
   set but QuicStreamHasPendingStreamData returns false. This is the third
   option from the issue, but keyed on QuicStreamHasPendingStreamData rather
   than QuicStreamSendCanWriteDataFrames — the former compares send offsets
   only, so it doesn't fire when data is pending but blocked by flow control or
   by the peer not yet allowing the stream. That's the concern raised in point 2
   above.

3. Regression test Misc.StreamZeroLengthSend fails on main at 4984c21 and
   passes with the fix. Each guard breaks the wedge independently, verified by
   disabling one at a time.

[Tanishq Sharma (Tani-shar)]

@microsoft-github-policy-service agree

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.94%. Comparing base (4984c21) to head (e769795).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/stream_send.c	75.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (75.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6248      +/-   ##
==========================================
- Coverage   85.28%   84.94%   -0.34%     
==========================================
  Files          60       60              
  Lines       18973    18977       +4     
==========================================
- Hits        16181    16120      -61     
- Misses       2792     2857      +65     

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