fix: contain chaff write failures during shutdown

[ptgamr]

Closing a PTY session while a keystroke-padding packet is in flight can let a TransportException escape the chaff coroutine and terminate the client process. We observed Android crashes with Connection closed: Socket closed and Packet writer stopped during tab shutdown. An admitted write can report a transport failure even after its coroutine has been cancelled.

Route these failures through the existing transportFailed path, consistent with the delivery workers fixed in #314. This closes unexpected failures normally and ignores connections already shutting down. Preserve ordinary CancellationException propagation and keep keystroke padding enabled.

Adds three deterministic regressions covering a failed chaff write, a write that fails after cancellation, and normal cancellation. The first two fail without the fix.

Validation: all 86 tests across KeystrokeObfuscationTest, SessionChannelTest, ForwardingChannelTest, ConnectionBackpressureTest, and PacketWriterTest pass. spotlessCheck and :sshlib:metalavaCheckCompatibility also pass.