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 (d0504a7).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5532    +/-   ##
========================================
  Coverage   93.44%   93.44%            
========================================
  Files         110      110            
  Lines       37329    37434   +105     
========================================
+ Hits        34883    34982    +99     
- Misses       2446     2452     +6     

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[vctqs1]

@mcollina yeah sure, i'll add unit-test for it

[vctqs1]

I updated it @mcollina could you please help review again, Thanks