fix(builtins/typed_array,iterable,generator,error,bigint,eval): convert panics to EngineError::Panic using js_expect

[KaustubhOG]

Part of #3241. It changes the following:

• builtins/typed_array/builtin.rs: converted 36 panics
• builtins/typed_array/object.rs: converted 4 panics
• builtins/iterable/async_from_sync_iterator.rs: converted 8 panics
• builtins/generator/mod.rs: converted 3 panics
• builtins/promise/mod.rs: converted 4 panics
• builtins/json/mod.rs: converted 2 panics
• builtins/error/aggregate.rs: converted 1 panic
• builtins/bigint/mod.rs: converted 1 panic
• builtins/eval/mod.rs: converted 1 panic

After this PR, core/engine/src/builtins/ is 98% panic-free.The remaining .expect() calls all fall into non-convertible categories:

Exhaustive match arms / logic invariants (no JsResult context):

• atomics/mod.rs — unreachable! on float variants in integer-only ops
• date/mod.rs, date/utils.rs — exhaustive match after prior validation
• temporal/ — exhaustive match arms
• intl/ — exhaustive match arms and logic invariants
• number/mod.rs, number/globals.rs — pure math helpers, no JsResult propagation path

Builtin initialization infrastructure:

• builder.rs — init-time invariants, no JsResult context
• error/type.rs — fn init() returning ()

Low-level buffer ops (intentional panic on corruption):

• array_buffer/utils.rs — raw slice index ops
• array_buffer/shared.rs — same
• atomics/futex.rs — OS threading node invariants

Non-JsResult contexts:

• promise/mod.rs:2770 — inside fn returning JsObject
• function/arguments.rs — inside fn returning ThinVec
• iterator_helper.rs — internal type invariants on self-owned objects

Test-only:

• uri/mod.rs — test function
• date/tests.rs — test file

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,935	49,935	0
Ignored	2,207	2,207	0
Failed	821	821	0
Panics	0	0	0
Conformance	94.28%	94.28%	0.00%

Tested main commit: 40f99905eefb4ac0542e4b7686425ffeb17ff09a
Tested PR commit: 6711953620dae7e1bb962762945e04d023a1caea
Compare commits: 40f9990...6711953

[codecov]

Codecov Report

❌ Patch coverage is 32.25806% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.98%. Comparing base (6ddc2b4) to head (6711953).
⚠️ Report is 866 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/typed_array/builtin.rs	4.16%	23 Missing ⚠️
core/engine/src/builtins/typed_array/object.rs	46.15%	7 Missing ⚠️
core/engine/src/builtins/promise/mod.rs	0.00%	4 Missing ⚠️
.../src/builtins/iterable/async_from_sync_iterator.rs	0.00%	3 Missing ⚠️
core/engine/src/builtins/generator/mod.rs	77.77%	2 Missing ⚠️
core/engine/src/object/builtins/jspromise.rs	0.00%	2 Missing ⚠️
core/engine/src/vm/opcode/await/mod.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5045       +/-   ##
===========================================
+ Coverage   47.24%   58.98%   +11.74%     
===========================================
  Files         476      563       +87     
  Lines       46892    62560    +15668     
===========================================
+ Hits        22154    36904    +14750     
- Misses      24738    25656      +918     

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[KaustubhOG]

Hey @jedel1043, addressed all 3 comments (does the looks good or any furtheree changes requierd). Also while I was at it, done with the remaining builtins/ panics in this same PR ( it got somewhere big but they were tiny (1-5 per file) scattered across promise/mod.rs, json/mod.rs and a couple more typed_array spots, i thought unecessary for a seprate PR to be raised.
.)

[jedel1043]

Thank you for this!

[KaustubhOG]

Remaining builtins/ panics audit — all invariants, zero convertible

File(s)	Hits	Reason
iterable/iterator_helper.rs	20	.expect("object type already verified") — downcast after explicit type-check gate. Spec-guaranteed unreachable.
atomics/tests.rs, intl/number_format/tests.rs, bigint/tests.rs, intl/locale/tests.rs, date/tests.rs, promise/tests.rs, math/tests.rs, function/tests.rs, array/tests.rs, array_buffer/tests.rs, number/tests.rs	76	All #[test] functions — panicking on failure is correct for tests. Not in scope for #3241.
number/mod.rs, number/globals.rs	15	Buffer iteration, radix validation, string slicing — all preceded by explicit bounds checks in the same function.
intl/locale/utils.rs	6	Inside #[cfg(test)] block — same as test files above.
builder.rs	6	Engine intrinsic bootstrap — failure here means a Boa bug, not a recoverable JS error.
string/mod.rs	5	Symbol-check done, length on new string, surrogate validated, i >= 0 asserted — all invariant-style.
array_buffer/utils.rs, array_buffer/shared.rs	8	Index bounds pre-validated by spec algorithm steps. shared.rs:357 — fixed-length check at caller.
intl/date_time_format/mod.rs	3	try_from_str on hardcoded BCP 47 literals ("gregory", "latn", "h12"). Compile-time constants, cannot fail.
uri/mod.rs	7	Lines 550–570: hardcoded byte literals in test fn. Line 589: test assertion context.
iterable/async_from_sync_iterator.rs	2	Spec-mandated invariants — prototype next and %Promise% return type guaranteed by spec's object model.
intl/segmenter/mod.rs, intl/number_format/options.rs, function/arguments.rs, error/type.rs, regexp/mod.rs, promise/mod.rs, array/mod.rs, atomics/futex.rs	17	Single-site invariants — range pre-checked, spec says "cannot fail", or unreachable-equivalent panics.
Total	165	0 convertible

All remaining panics fall into: test assertions, compile-time constant .expect, spec-guaranteed invariants, or engine bootstrap invariants. None are suitable for EngineError conversion.

[KaustubhOG]

[KaustubhOG]

Remaining builtins/ panics audit — all invariants, zero convertible

File(s) Hits Reason
iterable/iterator_helper.rs 20 .expect("object type already verified") — downcast after explicit type-check gate. Spec-guaranteed unreachable.
atomics/tests.rs, intl/number_format/tests.rs, bigint/tests.rs, intl/locale/tests.rs, date/tests.rs, promise/tests.rs, math/tests.rs, function/tests.rs, array/tests.rs, array_buffer/tests.rs, number/tests.rs 76 All #[test] functions — panicking on failure is correct for tests. Not in scope for #3241.
number/mod.rs, number/globals.rs 15 Buffer iteration, radix validation, string slicing — all preceded by explicit bounds checks in the same function.
intl/locale/utils.rs 6 Inside #[cfg(test)] block — same as test files above.
builder.rs 6 Engine intrinsic bootstrap — failure here means a Boa bug, not a recoverable JS error.
string/mod.rs 5 Symbol-check done, length on new string, surrogate validated, i >= 0 asserted — all invariant-style.
array_buffer/utils.rs, array_buffer/shared.rs 8 Index bounds pre-validated by spec algorithm steps. shared.rs:357 — fixed-length check at caller.
intl/date_time_format/mod.rs 3 try_from_str on hardcoded BCP 47 literals ("gregory", "latn", "h12"). Compile-time constants, cannot fail.
uri/mod.rs 7 Lines 550–570: hardcoded byte literals in test fn. Line 589: test assertion context.
iterable/async_from_sync_iterator.rs 2 Spec-mandated invariants — prototype next and %Promise% return type guaranteed by spec's object model.
intl/segmenter/mod.rs, intl/number_format/options.rs, function/arguments.rs, error/type.rs, regexp/mod.rs, promise/mod.rs, array/mod.rs, atomics/futex.rs 17 Single-site invariants — range pre-checked, spec says "cannot fail", or unreachable-equivalent panics.
Total 165 0 convertible
All remaining panics fall into: test assertions, compile-time constant .expect, spec-guaranteed invariants, or engine bootstrap invariants. None are suitable for EngineError conversion.

only for notes purpose will relocate all this (#5063 #5064 #5267) in #3241 and clear everything (that tracking progess bar ).