fix(pubsub): avoid deadlock by ensuring lease refresh timer runs on cq thread

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request addresses a self-deadlock issue in SubscriptionLeaseManagement::StartRefreshTimer by dispatching the lease refresh-timer continuation asynchronously via cq_.RunAsync instead of running it inline. This ensures that OnRefreshTimer always runs on a CompletionQueue thread without holding the mu_ lock. The associated unit tests have been updated to handle the deferred execution, and a new regression test has been added to verify this behavior. I have no feedback to provide.

[scotthart]

Resubmission of #16250

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.31%. Comparing base (d799c3f) to head (3123f01).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16297   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files        2223     2223           
  Lines      208086   208128   +42     
=======================================
+ Hits       192086   192131   +45     
+ Misses      16000    15997    -3     

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