fix: clear LiveKit key debounce timestamp on dispose

[marco-a-affinidi]

If a LiveKit backend is reused within makeKeyOnJoinDelay after dispose, preShareKey() may try to resend a recent key even though local key storage was already cleared. That null-key reuse path recurses through key generation and sending until it overflows the stack.

Reset _lastNewKeyTime on dispose so rejoining generates a fresh key.

[cursor]

PR Summary

Low Risk
Narrow VoIP E2EE lifecycle fix on dispose plus a regression test; no auth or broader API changes.

Overview
Fixes a stack overflow when the same LiveKitBackend is reused soon after dispose() and preShareKey() runs again.

dispose() already clears local encryption key storage and index state, but _lastNewKeyTime was left unchanged. Inside makeKeyOnJoinDelay, _makeNewSenderKey could still treat the last key as “recent” and try to resend it via _sendEncryptionKeysEvent. With no local key material, that path falls back to _makeNewSenderKey again and recurses until the stack overflows.

dispose() now resets _lastNewKeyTime to DateTime(1900), so a rejoin always goes through fresh key generation instead of resending a removed key.

Adds livekit_backend_test.dart to assert that after dispose and preShareKey on a new group call, latestLocalKeyIndex advances to 1 and the key provider sees indices [0, 1].

^{Reviewed by Cursor Bugbot for commit f7c27eb. Bugbot is set up for automated code reviews on this repo. Configure here.}

[CLAassistant]

All committers have signed the CLA.

[td-famedly]

good catch!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.60%. Comparing base (f9e6b68) to head (79c0abe).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2400      +/-   ##
==========================================
+ Coverage   59.29%   59.60%   +0.31%     
==========================================
  Files         161      161              
  Lines       20300    20305       +5     
==========================================
+ Hits        12037    12103      +66     
+ Misses       8263     8202      -61     

Files with missing lines	Coverage Δ
lib/src/voip/backend/livekit_backend.dart	27.18% <100.00%> (+27.18%)	⬆️

... and 2 files with indirect coverage changes

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[marco-a-affinidi]

good catch!

Thanks @td-famedly, looks like auto-merge did not work, do I need to do anything on my side?

[td-famedly]

@marco-a-affinidi looks like it needs a rebase so you are not outdated with main

[marco-a-affinidi]

Thanks @td-famedly, it's now rebased