fix(types): export MockAgent pending interceptor type

[tianrking]

Summary

• expose PendingInterceptor through the MockAgent namespace
• make pendingInterceptors() and its formatter reference the public namespaced types
• verify the public annotation in the TypeScript declaration tests

Why

MockAgent#pendingInterceptors() returned a declaration-private interface, so consumers could inspect the values but could not name the return element type in their own TypeScript APIs. The existing mock-agent type test had to duplicate that interface locally.

Validation

• npx tsd
• npx tsc test/imports/undici-import.ts --typeRoots ./types --noEmit
• npx tsc <all types/*.d.ts files> --noEmit --typeRoots ./types
• npm run lint -- --no-cache

Fixes #5563

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (87270e4) to head (61252ed).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5565   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files         110      110           
  Lines       37507    37507           
=======================================
  Hits        35059    35059           
  Misses       2448     2448           

☔ View full report in Codecov by Harness.
📢 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.