Fix incorrect reference start time for reclaiming session timeout

[lminiero]

This PR is an attempt to address the issue reported on the group, about a broken behaviour of our reclaim session functionality. As per the documentation, if a reclaim session timeout is configured, when a transport goes away you have X seconds to try and restore it from a different transport: the code was using the last activity as a start for the timeout, though, which was incorrect, since the last activity may precede the time we detect a transport to be gone, in some cases preventing any chance to reclaim due to early cleanups. This PR introduces a new timer that we track when the transport actually is detected as gone, and that we use in the related check.

Not tested yet, so please provide feedback on whether this works correctly and/or if it causes regressions.

[oleksandr-mihal-zultys]

I will test on 0.16.2 version

[lminiero]

I will test on 0.16.2 version

You'll need to port the fix to 0.x, then, as this PR is for master.

[oleksandr-mihal-zultys]

Works as expected.

session_timeout = 90
reclaim_session_timeout = 30

Last keep-alive:
[Tue May 12 05:45:21 2026] Got a keep-alive on session 74361634286954

LTE is turned OFF -> connection is closed:

[Tue May 12 05:45:45 2026] [WARN] [4893513815967762] Didn't receive audio for more than 1 seconds...
[Tue May 12 05:45:45 2026] [4893513815967762] Notifying that we are NOT receiving audio
[Tue May 12 05:45:45 2026] [4893513815967762] Sending event to transport...
[Tue May 12 05:45:45 2026] [WSS-0x7f5db0001dc0] WS connection down, closing
[Tue May 12 05:45:45 2026] [WSS-0x7f5db0001dc0] Destroying WebSocket client
[Tue May 12 05:45:45 2026] A janus.transport.websockets transport instance has gone away (0x7f5db0026a30)
[Tue May 12 05:45:45 2026]   -- Session 74361634286954 will be over if not reclaimed
[Tue May 12 05:45:45 2026]   -- Marking Session 74361634286954 as over
[Tue May 12 05:45:45 2026] [WSS-0x7f5db0001dc0]   -- closed

Connection is restored:

[Tue May 12 05:46:01 2026] [WSS-0x7f5db0002610] WebSocket connection opened from 127.0.0.1
[Tue May 12 05:46:01 2026] [WSS-0x7f5db0002610] WebSocket connection accepted
[Tue May 12 05:46:01 2026] [WSS-0x7f5db0002610]   -- Ready to be used!

Call is complete.
[Tue May 12 05:46:08 2026] Destroying session 74361634286954; 0x7f5de0005890