ci: set fail-fast to false on tests matrix to prevent platform blind spots

[RishavTiwari25]

This Pull Request fixes/closes #5152 .

It changes the following:

• Appends the explicit fail-fast: false property to the tests matrix strategy block within .github/workflows/rust.yml.
• Enhances developer diagnostic visibility by guaranteeing that every OS environment (macos, windows, ubuntu, ubuntu-arm) successfully completes its full test suite execution, even if a sibling platform fails first.
• Prevents GitHub Actions' default behavior from instantly force-cancelling the entire pipeline, eliminating "platform blind spots" where developers are unaware of platform-specific regressions.

[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: 055ee0958ce332f3f99af27536a7c6f9a91def27
Tested PR commit: 2bebcb66baabeea1e98122f1d1b1fecc7fa113d8
Compare commits: 055ee09...2bebcb6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.51%. Comparing base (6ddc2b4) to head (2bebcb6).
⚠️ Report is 890 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5153       +/-   ##
===========================================
+ Coverage   47.24%   59.51%   +12.27%     
===========================================
  Files         476      580      +104     
  Lines       46892    63181    +16289     
===========================================
+ Hits        22154    37602    +15448     
- Misses      24738    25579      +841     

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