chore: guard client[kUrl] in closeClientIfUnused loop

[vctqs1]

This relates to...

When a custom factory (e.g ProxyAgent) is used with Agent the dispatcher registered in kClients may not have kUrl set.
ProxyAgent extends DispatcherBase directly and never assigns this[kUrl] so client[kUrl] is undefined for every proxied origin key

Used Error.prepareStackTrace to surface the stack trace here . it was being suppressed because the frames originate from node:internal.

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[mcollina]

Thanks for opening a PR! Can you please add a unit test?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (3c662a5) to head (cc78087).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5532      +/-   ##
==========================================
- Coverage   93.44%   93.44%   -0.01%     
==========================================
  Files         110      110              
  Lines       37329    37329              
==========================================
- Hits        34883    34881       -2     
- Misses       2446     2448       +2     

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