firezone / boringtun / 34960873162

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fix: clear the passive keepalive when its session expires (#243)

A peer that has received data owes the other side a passive keepalive,
and that keepalive is encrypted on the current session. If the session
expires before the keepalive goes out, the send fails with
`NoCurrentSession`, the arm handling that swallows the error, and the
deadline behind it is left standing. Only an actual send clears it, so
every later poll reads it as due, announces it to the caller, and gets
nowhere. The caller is told to come straight back and nothing ever
changes.

A host that suspends walks right into this. The keepalive falls due
while nothing is running, and by the time the event loop resumes the
session is long gone. A responder has no other timer to fall back on, so
it burns a core until something else tears the connection down.

The deadline now goes with the session it depends on. Recovery is left
to the handshake timers, which is where it already belonged.

The simulation harness gains a `suspend` helper that advances time
without polling, so a test can put a peer in that state. The harness
already asserts that a deadline is never announced repeatedly without
firing, and that assertion is what catches the regression.

Co-authored-by: Claude <noreply@anthropic.com>

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