fix(string): cleanup code in String.prototype.repeat

[Tanush576]

Overview
This PR refactors the String.prototype.repeat implementation to strictly align with Section 22.1.3.16 of the ECMA-262 specification. The previous implementation used a generic error handling match; this update introduces explicit checks for Infinity and negative values as required by the abstract operations.

Spec Compliance Audit
I have mapped the implementation to the following specification steps:

Step 3: Let n be ? ToIntegerOrInfinity(count).

Step 4: If n < 0, throw a RangeError exception. (Implemented via explicit match)

Step 5: If n is +∞, throw a RangeError exception. (Implemented via PositiveInfinity check)

Changes
Replaced the catch-all error handling with an exhaustive match on the IntegerOrInfinity enum.

Added specific RangeError messages that distinguish between negative counts and infinite counts.

Used variable shadowing to simplify the logic flow from an Enum to a primitive i64 after validation.

Verification
I have verified the fix using a local test script across the following edge cases:

JavaScript
"abc".repeat(0); // Returns "" (Success)
"abc".repeat(-1); // Throws RangeError: count must be non-negative (Success)
"abc".repeat(Infinity); // Throws RangeError: count must be less than infinity (Success)
Note to Maintainers
I am a prospective GSoC 2026 applicant interested in the ECMAScript Conformance project. I chose this task to demonstrate my ability to translate ECMA-262 algorithms into idiomatic Rust using Boa's internal types. I am eager to contribute more to the engine's compliance suite!

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,070	-3
Ignored	2,072	2,072	0
Failed	818	821	+3
Panics	0	0	0
Conformance	94.54%	94.54%	-0.01%

Broken tests (3):

test/intl402/DateTimeFormat/prototype/toStringTag/toStringTag.js (previously Passed)
test/intl402/DateTimeFormat/prototype/toStringTag/toString.js (previously Passed)
test/intl402/Number/prototype/toLocaleString/throws-same-exceptions-as-NumberFormat.js (previously Passed)

Tested main commit: 0253916a4bca6a19be0a66f5b50842f3894d81dd
Tested PR commit: 91a7f8e1d0e3d8dd5148662dc6d58faeff5806d5
Compare commits: 0253916...91a7f8e

[Tanush576]

Hi @jedel1043 ! I hope you had a great weekend!
Just a gentle ping on this PR and my other open one (#5018). It looks like both are waiting for a maintainer to approve the GitHub Actions CI workflows. Could you please trigger the checks for them when you have a moment?
I'm standing by to make any updates or fix any formatting issues once the CI runs. Thank you!

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.15%. Comparing base (6ddc2b4) to head (91a7f8e).
⚠️ Report is 870 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/string/mod.rs	88.88%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5017       +/-   ##
===========================================
+ Coverage   47.24%   59.15%   +11.90%     
===========================================
  Files         476      563       +87     
  Lines       46892    62779    +15887     
===========================================
+ Hits        22154    37134    +14980     
- Misses      24738    25645      +907     

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[jedel1043]

Looks great!

[Tanush576]

Hey @jedel1043, I pulled down your web edits, fixed the minor syntax/doc comment error, ran cargo fmt, and pushed it back up. The CI checks are all green and good to go!

[jedel1043]

yeah that's what I get for trying to fix this through my phone 🤣🤣🤣
Thank you for the fix!