fix(number): fix Infinity casing in Number.prototype.toExponential

[ParthMozarkar]

I noticed Number.prototype.toExponential was failing the test262
return-values.js test and dug into why.

Infinity.toExponential() was returning "infinity" instead of "Infinity"
(-Infinity).toExponential() was returning "-infinity" instead of "-Infinity"

The issue was that the non-finite check was using JsString::from(this_num)
which relies on Rust's default float formatting that produces lowercase
"infinity".

Fixed it by explicitly returning the correct strings "Infinity",
"-Infinity" and "NaN" for non-finite values, matching what the
ECMAScript spec requires.

All existing tests pass.

Fixes #5243

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,545	50,545	0
Ignored	1,426	1,426	0
Failed	992	992	0
Panics	2	2	0
Conformance	95.43%	95.43%	0.00%

Tested main commit: 5b0f62ad860a2f5e2a4631ac2f280642cf7e21ea
Tested PR commit: 715134de84a62f05c1755b5281d557fd720a962e
Compare commits: 5b0f62a...715134d

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.80%. Comparing base (6ddc2b4) to head (715134d).
⚠️ Report is 917 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/number/mod.rs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5244       +/-   ##
===========================================
+ Coverage   47.24%   59.80%   +12.56%     
===========================================
  Files         476      582      +106     
  Lines       46892    63463    +16571     
===========================================
+ Hits        22154    37956    +15802     
- Misses      24738    25507      +769     

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