fix: Handle a visitor node restarting.

[bgrozev]

When the XMPP stream to a visitor node is re-established without being resumed, the node has lost the state of the MUCs that jicofo joined on it. Smack does not re-join them, and MultiUserChat.isJoined() keeps returning true (it is only myRoomJid != null, which a connection loss does not clear). Jicofo therefore believed it was still in the visitor MUC, received no presence from it, and kept redirecting new visitors there. Only a jicofo restart recovered from this.

Changes

• VisitorConnectionMonitor (new): one Smack ConnectionListener per visitor connection. It reports a reset on authenticated(resumed=false), i.e. when the state on the node is gone. A stream that resumes is left alone, which matches how the main room is handled.
• FocusManager.visitorConnectionReset() notifies the conferences.
• JitsiMeetConferenceImpl.visitorConnectionReset() discards the stale visitor room, terminates the visitors that were in it, leaves the room, and sends disconnect-vnode. The next redirect joins a new room.
• selectVisitorNode() does not re-use a node whose XMPP connection is down. The fallback for when all nodes are down does not change.
• selectVisitorNode() removes the map entry if the join fails, or if the room is discarded while we join it. Before this a failed join left an entry that jicofo treated as joined for the rest of the conference.

A visitor room that the server destroys had the same leak: the visitors in it kept a Participant and an endpoint on a bridge. Both paths now share the cleanup.

Tests

23 new tests in ConferenceUtilTest, ConferenceVisitorsTest and VisitorConnectionMonitorTest. With the changes to the production code reverted, 8 of them fail.

MockChatRoom.join() returned a relaxed mock, so mainRoomJid was never null and every conference in the tests looked like a breakout room. It now returns a real ChatRoomInfo.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.88%. Comparing base (7ebb13d) to head (143aac9).

Files with missing lines	Patch %	Lines
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	75.60%	8 Missing and 2 partials ⚠️
...o/src/main/kotlin/org/jitsi/jicofo/FocusManager.kt	0.00%	5 Missing ⚠️
.../main/kotlin/org/jitsi/jicofo/xmpp/XmppServices.kt	0.00%	3 Missing ⚠️
.../org/jitsi/jicofo/xmpp/VisitorConnectionMonitor.kt	88.88%	2 Missing ⚠️
...tlin/org/jitsi/jicofo/conference/ConferenceUtil.kt	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1307      +/-   ##
==========================================
+ Coverage   51.43%   52.88%   +1.44%     
==========================================
  Files         132      133       +1     
  Lines        8457     8511      +54     
  Branches     1290     1293       +3     
==========================================
+ Hits         4350     4501     +151     
+ Misses       3496     3384     -112     
- Partials      611      626      +15     

Files with missing lines	Coverage Δ
...g/jitsi/jicofo/conference/JitsiMeetConference.java	0.00% <ø> (ø)
...tlin/org/jitsi/jicofo/conference/ConferenceUtil.kt	48.38% <66.66%> (+20.80%)	⬆️
.../org/jitsi/jicofo/xmpp/VisitorConnectionMonitor.kt	88.88% <88.88%> (ø)
.../main/kotlin/org/jitsi/jicofo/xmpp/XmppServices.kt	0.00% <0.00%> (ø)
...o/src/main/kotlin/org/jitsi/jicofo/FocusManager.kt	37.43% <0.00%> (-1.08%)	⬇️
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	47.35% <75.60%> (+9.20%)	⬆️

... and 7 files with indirect coverage changes

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