feat(error): harden JsNativeError public API surface

[alienx5499]

This Pull Request fixes/closes #4981.

It changes the following:

• Make JsNativeError::kind a private field instead of pub.
• Add a JsNativeError::kind(&self) -> &JsNativeErrorKind accessor as the stable API.
• Update internal usages (engine tests, runtime tests, URI tests, tester helpers, docs/examples) to call kind() instead of reading the field directly.

Testing:

• cargo test -p boa_engine

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,901	49,901	0
Ignored	2,222	2,222	0
Failed	840	840	0
Panics	0	0	0
Conformance	94.22%	94.22%	0.00%

Tested main commit: a49348586d90b790936dc6ab651da6aff3f60e67
Tested PR commit: 07f3eaacdf3fb60b47c4b250c51d2de7624e0d4b
Compare commits: a493485...07f3eaa

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.39%. Comparing base (6ddc2b4) to head (07f3eaa).
⚠️ Report is 808 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/tester/src/exec/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5012       +/-   ##
===========================================
+ Coverage   47.24%   58.39%   +11.14%     
===========================================
  Files         476      559       +83     
  Lines       46892    61406    +14514     
===========================================
+ Hits        22154    35855    +13701     
- Misses      24738    25551      +813     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 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.

[jedel1043]

Cool, thank you!