Make sure to check if the server is closed in etcd2topo

[GrahamCampbell]

Description

Fixes an issue during VTGate shutdown where s.cli is set to nil in Close() method while concurrent coroutines still try to access it, causing nil pointer dereference panics.

This fix replaces a panic (which crashes the process) with a graceful shutdown signal (which components handle cleanly). The error becomes part of the normal shutdown flow, not a failure condition.

Related Issue(s)

Fixes #18350.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

None.

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.48%. Comparing base (2520f7f) to head (e33b1f9).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18352      +/-   ##
==========================================
- Coverage   67.49%   67.48%   -0.01%     
==========================================
  Files        1603     1603              
  Lines      262428   262470      +42     
==========================================
+ Hits       177114   177131      +17     
- Misses      85314    85339      +25     

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