firezone / sanstun / 38812674931

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refactor!: return each received message's reply directly (#114)

`Tunnel::begin_decapsulate` took any `ReceivedMessage` and returned
`Ok(None)` for handshakes, whose actual output (the response, or the
keepalive that confirms the session) was queued as an event. Callers
already match on `ReceivedMessage` to route it, so each variant now has
its own method that returns exactly what it produces:
`handle_handshake_initiation` returns the response,
`handle_handshake_response` returns the confirming keepalive,
`handle_cookie_reply` returns nothing, and `begin_decapsulate` takes
only `TransportData`.

The rule is that an input returns its direct reply and only timers
produce events. Every event now carries the datagram to send, including
timer-driven keepalives, which removes the `Keepalive` token and
`encapsulate_keepalive`.

`begin_decapsulate` now borrows the tunnel immutably, so the compiler
guarantees that an unauthenticated data message cannot change a tunnel;
that only happens once it is authenticated, in `commit_decapsulate`.

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

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