fix(string): ensure separator is an object before calling Symbol.split

[amrkhaled104]

This Pull Request fixes #4663

Changes:

• Added a check to ensure the separator is an Object before looking for the @@split method.
• This prevents the engine from calling custom split functions added to primitive prototypes (like BigInt.prototype).
• Updated the logic in core/engine/src/builtins/string/mod.rs

Validation:

• I ran the official Test262 suite for String.prototype.split.
• Before fix: 116/120 tests passed (4 failed).
• After fix: 120/120 tests passed (100% conformance).

Test Results Screenshot:

I have expanded the fix to include other string methods that were also incorrectly accessing Symbol methods on primitive separators.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,472	49,487	+15
Ignored	2,249	2,249	0
Failed	1,141	1,126	-15
Panics	0	0	0
Conformance	93.59%	93.62%	+0.03%

Fixed tests (15):

test/built-ins/String/prototype/matchAll/cstm-matchall-on-bigint-primitive.js (previously Failed)
test/built-ins/String/prototype/matchAll/cstm-matchall-on-number-primitive.js (previously Failed)
test/built-ins/String/prototype/matchAll/cstm-matchall-on-string-primitive.js (previously Failed)
test/built-ins/String/prototype/search/cstm-search-on-bigint-primitive.js (previously Failed)
test/built-ins/String/prototype/search/cstm-search-on-number-primitive.js (previously Failed)
test/built-ins/String/prototype/search/cstm-search-on-boolean-primitive.js (previously Failed)
test/built-ins/String/prototype/search/cstm-search-on-string-primitive.js (previously Failed)
test/built-ins/String/prototype/match/cstm-matcher-on-bigint-primitive.js (previously Failed)
test/built-ins/String/prototype/match/cstm-matcher-on-boolean-primitive.js (previously Failed)
test/built-ins/String/prototype/match/cstm-matcher-on-number-primitive.js (previously Failed)
test/built-ins/String/prototype/match/cstm-matcher-on-string-primitive.js (previously Failed)
test/built-ins/String/prototype/split/cstm-split-on-boolean-primitive.js (previously Failed)
test/built-ins/String/prototype/split/cstm-split-on-bigint-primitive.js (previously Failed)
test/built-ins/String/prototype/split/cstm-split-on-number-primitive.js (previously Failed)
test/built-ins/String/prototype/split/cstm-split-on-string-primitive.js (previously Failed)

[jedel1043]

Kind of unrelated, but now that you're touching this part of the code, can you change this to use let chains and &&s?
Something like if !separator.blah() && let Some(separator_obj) = blah

Also kind of applies to the rest of the added lines since I see that a lot of them are just two nested ifs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.63%. Comparing base (6ddc2b4) to head (83c102f).
⚠️ Report is 678 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4664      +/-   ##
==========================================
+ Coverage   47.24%   56.63%   +9.39%     
==========================================
  Files         476      548      +72     
  Lines       46892    60011   +13119     
==========================================
+ Hits        22154    33990   +11836     
- Misses      24738    26021    +1283     

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