CI: add workflow to cleanup PR caches on close

[RishavTiwari25]

This Pull Request fixes/closes #5128 .

It changes the following:

• Adds .github/workflows/cleanup-caches.yml workflow.
• Automatically deletes GitHub Actions caches scoped specifically to a pull request branch when that PR is closed or merged.
• Prevents orphaned PR caches from accumulating and exceeding the repository's 10 GB limit, reducing cache eviction churn.
• Uses the gh actions-cache delete CLI securely with actions: write scope.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,073	0
Ignored	2,072	2,072	0
Failed	818	818	0
Panics	0	0	0
Conformance	94.54%	94.54%	0.00%

Tested main commit: 70409a5052984325dccfdc5f6520818568a81f39
Tested PR commit: dbab35bbce94f1930b7fde14ded227c8528460cb
Compare commits: 70409a5...dbab35b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.31%. Comparing base (6ddc2b4) to head (dbab35b).
⚠️ Report is 878 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5129       +/-   ##
===========================================
+ Coverage   47.24%   59.31%   +12.07%     
===========================================
  Files         476      563       +87     
  Lines       46892    62795    +15903     
===========================================
+ Hits        22154    37249    +15095     
- Misses      24738    25546      +808     

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