dkg: fix flaky TestSyncFlow test

[pinebit]

Fix flaky TestSyncFlow test, specifically the three_connect_one_disconnect case.

Problem

The test was probably flaky because it relied on passive TCP timeouts to detect peer disconnections. When a peer was "stopped" (via context cancellation), other peers wouldn't detect the disconnection quickly, causing the test to hang or timeout intermittently in CI.

Solution

Added explicit libp2p connection closure when simulating peer disconnection in the test. When the test stops a peer, it now iterates through all other running peers and forcibly closes the libp2p connection to the stopped peer using host.Network().ClosePeer(). This ensures immediate failure detection without relying on timeouts.

category: refactor
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.34%. Comparing base (1c29397) to head (54558c4).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4167      +/-   ##
==========================================
+ Coverage   56.29%   56.34%   +0.04%     
==========================================
  Files         245      245              
  Lines       31246    31246              
==========================================
+ Hits        17591    17606      +15     
+ Misses      11337    11328       -9     
+ Partials     2318     2312       -6     

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