Add Agent.restartIce(): in-place ICE restart on an existing Agent

[bgrozev]

Summary

Adds Agent.restartIce(): an in-place ICE restart on an already-established Agent, re-running
connectivity checks against new remote credentials (and candidates) without creating a new Agent. Local
credentials, local candidates and sockets are unchanged.

The currently selected pair is kept in use for sending until a new pair is nominated
(make-before-break): the check list and valid list are reset and re-run, but the selected pair is only
replaced once a new nomination is confirmed for the restarting component.

This required relaxing two set-once guards that normally assume nomination happens only once per
component, both gated on a new Component.isIceRestarting flag so ordinary ICE processing is unaffected:

• CheckList.handleNominationConfirmed() now swaps the selected pair instead of no-oping when the
  component is restarting.
• ConnectivityCheckClient.processSuccessResponse() now also confirms a nomination while a selected pair
  already exists, if the component is restarting (otherwise the successful check is treated as a
  keepalive and nominationConfirmed() is never called, so the swap never happens and the Agent never
  returns to Completed).

Also un-stops the connectivity check client and cancels a pending or already-fired termination, since a
steady-state Agent has its check client stopped and enters Terminated a few seconds after completion.

This is a leaf dependency for an in-place ICE restart feature spanning jitsi-videobridge and
lib-jitsi-meet (same branch name, ice-restart, in both).

Includes a new test, IceRestartTest, covering the re-arm invariants over a two-agent loopback
connection.

[codecov]

Codecov Report

❌ Patch coverage is 81.48148% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.78%. Comparing base (6da2b08) to head (add8fff).

Files with missing lines	Patch %	Lines
src/main/java/org/ice4j/ice/Agent.java	71.42%	5 Missing and 1 partial ⚠️
src/main/java/org/ice4j/ice/CheckList.java	85.71%	1 Missing and 1 partial ⚠️
...in/java/org/ice4j/ice/ConnectivityCheckClient.java	83.33%	0 Missing and 1 partial ⚠️
src/main/java/org/ice4j/ice/IceMediaStream.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #310      +/-   ##
============================================
+ Coverage     36.44%   36.78%   +0.34%     
- Complexity     1151     1174      +23     
============================================
  Files           166      166              
  Lines         11037    11046       +9     
  Branches       1629     1635       +6     
============================================
+ Hits           4022     4063      +41     
+ Misses         6496     6458      -38     
- Partials        519      525       +6     

Files with missing lines	Coverage Δ
src/main/java/org/ice4j/ice/Component.java	40.89% <100.00%> (+2.49%)	⬆️
...in/java/org/ice4j/ice/ConnectivityCheckClient.java	57.70% <83.33%> (-1.39%)	⬇️
src/main/java/org/ice4j/ice/IceMediaStream.java	65.04% <88.88%> (+1.44%)	⬆️
src/main/java/org/ice4j/ice/CheckList.java	60.67% <85.71%> (+1.74%)	⬆️
src/main/java/org/ice4j/ice/Agent.java	57.18% <71.42%> (+3.08%)	⬆️

... and 16 files with indirect coverage changes

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6da2b08...add8fff. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.