Improve zombie client connection detection

[harrylin98]

Preparatory cleanup extracted from the throttling PR: https://github.com/valkey-io/valkey/pull/4356/changes#diff-b95cee2ea24f3f4245c20ba624dd69c6b741fe7d63e64a29e451e4f09b536abeR426

• Rename connSocketIsClosing → connTcpSocketIsClosing
• Move the "has active handler" check from the connection layer up to clientsCronTcpIsClosing
• Use offsetof-based length validation for getsockopt instead of requiring the full struct size
• Run zombie detection earlier in clientsCron (skip buffer resize on dead clients)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.40%. Comparing base (50ec936) to head (57ad97b).

Additional details and impacted files

@@             Coverage Diff              @@
##           forkless    #4465      +/-   ##
============================================
- Coverage     79.82%   79.40%   -0.43%     
============================================
  Files           175      175              
  Lines         94843    93223    -1620     
============================================
- Hits          75712    74021    -1691     
- Misses        19131    19202      +71     

Files with missing lines	Coverage Δ
src/connection.h	88.67% <ø> (ø)
src/server.c	89.75% <100.00%> (+<0.01%)	⬆️
src/socket.c	94.68% <100.00%> (+0.53%)	⬆️
src/tls.c	17.64% <ø> (ø)

... and 25 files with indirect coverage changes

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[JimB123]

If this isn't critical specifically for the forkless branch, I'd recommend keeping this with the throttling branch and resolve during the merge.

If you concur, please close the PR