http2: take the credit wait before the flush that provokes the credit

[MDA2AV]

The bug

A streamed HTTP/2 writer that runs out of window flushes what it has staged before parking, because WINDOW_UPDATE is what a peer sends once it has consumed DATA — parking with those bytes still buffered waits for a message that the wait itself prevents.

That flush is an await, and the WINDOW_UPDATE it exists to provoke can arrive while the writer is still inside it:

credit == 0
  -> await FlushOutboundAsync()      <- WINDOW_UPDATE lands here
  -> WaitForSendCreditAsync(stream)  <- registers only now

ReleaseCreditWaiters runs during the flush, finds no waiter for the stream, and drops the wake-up. The writer then registers and parks on a message that has already been and gone. The response stops mid-body and the client stalls until its own timeout.

Registering before the flush closes it: credit arriving during the flush completes that task, and the await after it returns at once. It is the only call site.

How it showed up

Intermittent 10-second TaskCanceledException timeouts in GenHTTP's acceptance suite, on the two tests that push a body past the initial 65535-byte window. Roughly one run in eight, and never in isolation — 22 consecutive runs of those tests alone passed, including pinned to two CPUs.

The test

h2 body: credit arriving while the pre-park flush is in flight is not lost drives it deterministically. Two details matter, both learned the hard way:

• It credits while a flush is still in flight, then releases. DrainWithCredit credits only once PendingFlushes is 0 — after the flush has been released and the writer is already registered — which is why the existing coverage never saw this.
• It credits 8 KiB at a time. An earlier draft credited 1 MiB up front and passed against the bug, because a window big enough to carry the rest of the body lets the writer finish without ever parking.

It fails on main (DATA with END_STREAM for stream 1 never arrives) and passes with the fix.

Validation

Unit 43, E2E 180, Chaos 47, Http 44, File 4 — 0 failed.