Every ending path (peer Disconnect, local disconnect, timeout, refusal or
reset) must drop the writes it owed, so that a later session neither
retransmits nor delivers them; Rust adds the receipt after a local
disconnect and a reconnect. A flush that starts after an ending, or that
started before one and sees a Connect begin a new session, must fail. The
C# and Python flush tests use a second thread for the mid-wait reconnect.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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