Update test262 to 673e9bac and fix newly-revealed issues

[lahma]

Summary

Bumps the Test262 suite hash from 2b2ecead to 673e9bac, which brings stage-4 spec updates for the immutable-arraybuffer proposal and a stricter ArrayBuffer.prototype.sliceToImmutable algorithm. Twelve previously failing test cases now pass:

• built-ins/ArrayBuffer/prototype/sliceToImmutable/this-shrinks.js
• built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js
• built-ins/TypedArrayConstructors/from/custom-ctor-returns-immutable-arraybuffer.js
• built-ins/TypedArrayConstructors/of/custom-ctor-returns-immutable-arraybuffer.js
• built-ins/Uint8Array/prototype/setFromBase64/throws-when-target-is-backed-by-immutable-arraybuffer.js
• built-ins/Uint8Array/prototype/setFromHex/throws-when-target-is-backed-by-immutable-arraybuffer.js

Fixes

• ArrayBufferPrototype.SliceToImmutable rewritten to the latest spec. first/final are still computed from the original len, but the detach check now precedes the bounds check, and a RangeError is thrown when the buffer has shrunk below final during argument coercion.
• TypeConverter.ToNumber(string) guards against empty input after TrimEx. The pre-trim string.IsNullOrWhiteSpace check misses BOM () and other characters that the spec-compliant TrimEx removes — this previously caused an IndexOutOfRangeException on whitespace-padded empty strings (the new argument-coercion.js reaches that path).
• Uint8Array.prototype.setFromBase64 / setFromHex call AssertNotImmutable() on the viewed buffer before writing, per the new spec step.
• %TypedArray%.from / .of call AssertNotImmutable() on a custom constructor's result before populating it (the write mutability intent in TypedArrayCreateFromConstructor).
• TypedArrayConstructor.IterableToList now passes the already-fetched iterator method through to GetIterator instead of re-reading Symbol.iterator on the source.

CI host crash fix

A FinalizationRegistry cleanup callback can throw arbitrary exceptions (e.g. TimeoutException when engine constraints trip), and they were escaping the GC finalizer thread and aborting the whole test process — visible on this run. The callback invocation is now wrapped in a swallow-all try/catch, matching the spec's intent that these callbacks run in an isolated Job.

Test plan

• Full test262 suite green: 99013 passed, 0 failed, 133 skipped.
• Jint.Tests green (2885 passed).
• Jint.Tests.PublicInterface green (79 passed).
• Jint.Tests.CommonScripts green (28 passed).

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