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 93 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

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[PickBas]

Closing this PR, as a similar PR has already been merged that also resolves #64322.