test_runner: align MockTimers timeout API

[Han5991]

Summary

Align MockTimers timeout handles with the public Timeout API by exposing close() and [Symbol.dispose]() on fake setTimeout() results.

Problem

When test.mock.timers.enable() is active, setTimeout() returns a mock timeout handle that does not implement Timeout.close() or Timeout[Symbol.dispose](). That breaks disposal-based patterns such as DisposableStack.use(timeout).

Root cause

MockTimers used its own Timeout wrapper with only ref(), unref(), refresh(), and hasRef(), so the fake timeout handle diverged from the documented timeout API.

Changes

• add Timeout.close() to the MockTimers timeout wrapper
• add Timeout[Symbol.dispose]() to the MockTimers timeout wrapper
• route both methods through the existing timer clear path
• add targeted coverage for both disposal entry points

Validation

• reproduced the issue with out/Release/node before the change
• python3 tools/test.py test/parallel/test-runner-mock-timers.js
• verified the built out/Release/node now reports dispose function, close function, and allows DisposableStack.use(timeout)

Fixes: #62815

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.66%. Comparing base (58a8e1d) to head (eea8f04).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62820      +/-   ##
==========================================
- Coverage   89.69%   89.66%   -0.03%     
==========================================
  Files         706      706              
  Lines      218222   218235      +13     
  Branches    41768    41772       +4     
==========================================
- Hits       195731   195683      -48     
- Misses      14411    14455      +44     
- Partials     8080     8097      +17     

Files with missing lines	Coverage Δ
lib/internal/test_runner/mock/mock_timers.js	98.71% <100.00%> (+0.02%)	⬆️

... and 34 files with indirect coverage changes

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