fix: preserve MockAgent interceptors for legacy global fetch

[pacocartones]

Summary

Fixes the regression where MockAgent interceptors are bypassed by Node.js global fetch after setGlobalDispatcher().

Dispatcher1Wrapper correctly forces legacy consumers onto HTTP/1.1 with allowH2: false. The internal Agent then uses its #http1-only pool key, but MockAgent only initialized mocks under the normal origin key. That allowed the internal agent to create a plain pool and make a real request even after disableNetConnect().

This creates the HTTP/1.1-only mock dispatcher before delegation and shares the registered interceptors with the normal-origin mock dispatcher. The legacy bridge keeps its HTTP/1.1 guarantee; only the mock selection path is fixed.

Regression coverage

The new subprocess test uses Node.js global fetch, a MockAgent, and disableNetConnect(). It expects the configured JSON response, so it fails if the request escapes the mock.

Validation

• npm run lint -- lib/mock/mock-agent.js test/node-test/global-dispatcher-version.js
• node --test test/node-test/global-dispatcher-version.js
• node --test test/mock-pool.js
• git diff --check

Fixes #5036.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (c8d80e6) to head (5499b8c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5648      +/-   ##
==========================================
+ Coverage   93.42%   93.44%   +0.01%     
==========================================
  Files         110      110              
  Lines       38710    38722      +12     
==========================================
+ Hits        36165    36182      +17     
+ Misses       2545     2540       -5     

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[pacocartones]

CI triage for the current head:\n\n- Both CodeQL analyses fail during autobuild because the repository configuration requires 4.37.3 while the runner is using 4.36.2; this is before analysis of this PR's change.\n- The Node 24 coverage job has one failing file: est/http2-request-never-settles.js. It is outside this diff and is the known high-churn HTTP/2 test being isolated in #5643.\n\nThe focused MockAgent regression and coverage remain green. I have not retried the unrelated jobs to avoid adding CI noise.

[mcollina]

lgtm