Fix clientTtl cleanup race in Agent

[mcollina]

Summary

• skip closing destroyed dispatchers when Agent removes idle clients
• avoid closing destroyed pool clients during pool shutdown
• add regression test for issue Unhandled rejection ClientDestroyedError when using Agent with clientTtl #4806

Testing

• node test/issue-4806.js

Fixes #4806

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (ff6bc5a) to head (71136ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4807      +/-   ##
==========================================
+ Coverage   93.23%   93.24%   +0.01%     
==========================================
  Files         109      109              
  Lines       34100    34108       +8     
==========================================
+ Hits        31793    31805      +12     
+ Misses       2307     2303       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.