fix: prevent overlapping group call force rejoins

[marco-a-affinidi]

When GroupCallSession.onMemberStateChanged() observed the local membership disappear while the call was still in entered, concurrent updates could trigger the force-rejoin path more than once before room state caught up. That led to repeated sendMemberStateEvent() and preShareKey() work, which could cascade into redundant key generation and unstable local participant state.

This change extracts the recovery flow into _forceRejoin(), shares a single in-flight rejoin across concurrent callers, and clears the stale local participant cache before awaiting the resend path.

The test coverage was also tightened with focused GroupCallSession regressions that prove both behaviors: stale local state is cleared until room state catches up, and concurrent updates do not start multiple force rejoins.

[cursor]

PR Summary

Medium Risk
Touches group-call membership recovery and key pre-sharing when server/client state diverge; behavior is well covered by new tests but affects live VoIP stability paths.

Overview
When room membership shows the local user left while the session is still entered, onMemberStateChanged used to run the recovery path inline on every concurrent update—repeated sendMemberStateEvent() and preShareKey() and unstable local participant state.

The recovery flow is now _forceRejoin(), guarded by a single _forceRejoinInProgress future so overlapping callers share one rejoin. Before waiting on the resend/key path, the stale local entry is removed from _participants so later updates do not re-trigger force rejoin; on failure while still entered, that participant is restored so a retry can run. Delayed-event canceller cleanup is unchanged in behavior but lives in the helper.

test/group_call_session_test.dart adds regressions for cache clearing, canceller teardown, retry after failure, single concurrent rejoin, and shared failure for waiters.

^{Reviewed by Cursor Bugbot for commit 71f567a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 2 potential issues.

^{Bugbot Autofix is ON, but it could not run because the branch was deleted or merged before autofix could start.}

^{Reviewed by Cursor Bugbot for commit 4ff45e3. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.70%. Comparing base (53a6a79) to head (71f567a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2406      +/-   ##
==========================================
+ Coverage   59.62%   59.70%   +0.07%     
==========================================
  Files         161      161              
  Lines       20329    20342      +13     
==========================================
+ Hits        12121    12145      +24     
+ Misses       8208     8197      -11     

Files with missing lines	Coverage Δ
lib/src/voip/group_call_session.dart	84.70% <100.00%> (+5.36%)	⬆️

---

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 53a6a79...71f567a. Read the comment docs.