docs: clarify MockAgent pool usage

[vibhor-aggr]

Summary

Closes #996.

Clarifies that setGlobalDispatcher(mockAgent) only affects Undici APIs that use the global dispatcher and does not monkeypatch separately constructed Pool or Client instances. Adds a small example showing the recommended testing pattern: pass the MockPool returned by mockAgent.get(origin) into code that accepts a pool.

Verification

PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH npm run lint
PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH node --input-type=module <new MockPool example>
git diff --check

Results: lint passed, the MockPool example returned 200, and diff check passed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (c7c7e7f) to head (9126497).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5542   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files         110      110           
  Lines       37376    37376           
=======================================
+ Hits        34928    34930    +2     
+ Misses       2448     2446    -2     

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[mcollina]

lgtm