src: avoid use-after-close of delayed task scheduler async handle

[PickBas]

src: avoid use-after-close of delayed task scheduler async handle

A worker thread posting a delayed task while the platform is shutting down could call uv_async_send() on the scheduler's async handle after Stop() began closing it, aborting with Assertion failed: !(handle->flags & UV_HANDLE_CLOSING).

This adds a stopped_ flag, guarded by the existing task queue lock, so late PostDelayedTask() calls are dropped instead of racing the handle close. Includes a cctest reproducing the shutdown race.

Fixes: #64322

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.23%. Comparing base (4140d4c) to head (80d2173).
⚠️ Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_platform.cc	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64492      +/-   ##
==========================================
- Coverage   90.25%   90.23%   -0.02%     
==========================================
  Files         741      741              
  Lines      241216   241692     +476     
  Branches    45442    45552     +110     
==========================================
+ Hits       217713   218101     +388     
- Misses      15079    15101      +22     
- Partials     8424     8490      +66     

Files with missing lines	Coverage Δ
src/node_platform.cc	76.04% <75.00%> (-0.20%)	⬇️

... and 65 files with indirect coverage changes

🚀 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.