Backport #4216 to 4.x: make Array.prototype.concat spread host arrays and lists

[lahma]

This backports one fix from main to 4.x. A second candidate was skipped because 4.x does not have the code it changes.

Commit	Main PR	What it fixes
5a0b45b	#4216 (0a6288a)	Array.prototype.concat nested host arrays and lists instead of spreading them (#4201). Since 4.14, ArrayConversionMode.LiveView has been the default, and under it engine.SetValue("a", new[] { x }); a.concat(b) returned a two-element array holding the two wrappers. The same happened with any List<T>.

Adapted for 4.x

• On 4.x, IsConcatSpreadable falls back to IsArray(), not main's IsSpecArray(). The fix keeps that call and adds the same wrapper test after it: IsArray() || this is ObjectWrapper { IsArrayLike: true, HasIndexedElements: true }. On 4.x, ObjectWrapper.IsArrayLike and HasIndexedElements match main, including the TypeResolver member-filter gate in HasIndexedElements. A queue, a stack or a set still is not spread. A JsArray still never reaches the type test.
• The test file moved to Jint.Tests/Runtime/ClrCollectionConcatTests.cs (4.x has no Runtime/Interop folder), and its [Test] attributes became xUnit [Fact]s. 4.x has no SpecAnchors.txt, so main's anchor-list change is not included.
• No public API or default changes.

Evidence

The seven tests in ClrCollectionConcatTests were run first without the fix. ConcatSpreadsTwoClrArraysOfExpandoObjects is the host code from #4201, as written in the issue.

TFM	Unfixed 4.x	With the fix
net10.0	6 failed, 1 passed	7 passed
net472	6 failed, 1 passed	7 passed

The test that passes without the fix is ConcatDoesNotSpreadACollectionWithoutIndexedElements. It checks that a Queue<T> stays a single element, so it should pass both before and after the fix.

Skipped

• Check MaxOutputCharacters against a string's length before ConvertResult copies it #4185 (53f38d9), "Check MaxOutputCharacters against a string's length before ConvertResult copies it". 4.x has none of the code it changes: git grep MaxOutputCharacters upstream/4.x and git grep ConvertResult upstream/4.x both find nothing, and there is no ResultConverter. MaxOutputCharacters was added on main by Bound result conversion, JSON serialization, and error rendering #3046, which is part of the Bound parser source length and AST size #3037-Add hardened untrusted-code profile #3060 security stack that 4.x does not have. Without that limit there is nothing to check earlier.

Test totals (Release, after the fix)

• Jint.Tests on net10.0: 7662 passed, 4 skipped, 0 failed
• Jint.Tests on net472: 7577 passed, 4 skipped, 0 failed
• Jint.Tests.PublicInterface on net10.0: 1903 passed, 9 skipped, 0 failed
• Jint.Tests.PublicInterface on net472: 1895 passed, 9 skipped, 0 failed
• Jint.Tests.Test262, full run on net10.0: 102509 passed, 175 skipped, 0 failed

The public API Verify snapshots are unchanged; the working tree was clean after the runs.

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