test: restore global dispatcher after fetch tests

[mcollina]

Summary

• restore the previous global dispatcher after tests that override it
• avoid leaking aggressive Agent settings across fetch test files

Testing

• not run (lint runs on commit via hook)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (352c691) to head (e7d1005).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4790      +/-   ##
==========================================
+ Coverage   93.25%   93.26%   +0.01%     
==========================================
  Files         109      109              
  Lines       34024    34024              
==========================================
+ Hits        31729    31733       +4     
+ Misses       2295     2291       -4     

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