fix(builtins): correct toPrecision output for subnormal floats

[HiteshShonak]

This Pull Request fixes/closes #5225.

It changes the following:

• When format!("{this_num:.100}") collapses a subnormal float to all zeros, fall back to scientific notation formatting to correctly extract the mantissa and exponent.
• Add regression tests for Number.MIN_VALUE.toPrecision(1) and toPrecision(2).

Testing:

cargo test -p boa_engine number -- --nocapture

Spec reference: https://tc39.es/ecma262/#sec-number.prototype.toprecision

[Copilot]

Pull request overview

Fixes Number.prototype.toPrecision for extremely small (including subnormal) f64 values where fixed-point formatting at 100 fractional digits collapses to all zeros, by falling back to scientific formatting to derive the correct mantissa/exponent.

Changes:

• Detects the “all zeros” fixed-point formatting case and derives digits/exponent from scientific notation instead.
• Adds regression tests covering Number.MIN_VALUE.toPrecision(1) and (2) to prevent reintroducing the bug.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
core/engine/src/builtins/number/mod.rs	Adds scientific-notation fallback when fixed formatting loses all significant digits, fixing exponent/mantissa extraction.
core/engine/src/builtins/number/tests.rs	Adds regression coverage for Number.MIN_VALUE precision formatting.

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[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,732	50,733	+1
Ignored	1,426	1,426	0
Failed	805	804	-1
Panics	0	0	0
Conformance	95.79%	95.79%	+0.00%

Fixed tests (1):

test/staging/sm/Number/toPrecision-values.js (previously Failed)

Tested main commit: 2437b6704c4315f6ec8f0766bf229fba23e0e61e
Tested PR commit: 149da84d4b5fe96d737ea061fcb698c2fe42ba28
Compare commits: 2437b67...149da84

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.71%. Comparing base (6ddc2b4) to head (149da84).
⚠️ Report is 919 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5226       +/-   ##
===========================================
+ Coverage   47.24%   59.71%   +12.47%     
===========================================
  Files         476      589      +113     
  Lines       46892    63415    +16523     
===========================================
+ Hits        22154    37869    +15715     
- Misses      24738    25546      +808     

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