Verify Jint's host contracts on every test run

The scripting integration defines several Jint extension points: the
ContentWrapper objects override GetOwnProperty, and the engine trusts the
answer without re-verifying it on the hot path. A hook that contradicts
another therefore fails silently in production - a key vanishes from every
enumeration, or a read resolves on the prototype for a property that
exists - which is the class of bug no assertion in this repository would
catch.

Jint 4.15.3 exposes its host-contract verifiers to the shipped Release
package through an AppContext switch, where before they were compiled out
unless you built the engine from source in Debug. A module initializer sets
it for this test assembly, so the verifiers run against the same NuGet
package production uses and report a violation as an ordinary test failure.
It must be set before the first use of any Jint type, which is exactly what
a module initializer guarantees.

Confirmed live rather than assumed: with a deliberately wrong
ProbeOwnProperty the run fails with "ContentFieldObject.ProbeOwnProperty
answered 'iv' with Missing but its GetOwnProperty reports Enumerable".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016uV6H9cTntzsoKiaJRBn4f

backend/tests/Squidex.Domain.Apps.Core.Tests/TestHelpers/JintHostContractVerification.cs

@@ -0,0 +1,35 @@
+// ==========================================================================
+//  Squidex Headless CMS
+// ==========================================================================
+//  Copyright (c) Squidex UG (haftungsbeschraenkt)
+//  All rights reserved. Licensed under the MIT license.
+// ==========================================================================
+
+using System.Runtime.CompilerServices;
+
+namespace Squidex.Domain.Apps.Core.TestHelpers;
+
+/// <summary>
+/// Turns on Jint's host-contract verifiers for this test assembly.
+/// </summary>
+/// <remarks>
+/// The scripting integration defines several Jint extension points - the ContentWrapper objects override
+/// GetOwnProperty and ProbeOwnProperty, and the engine trusts both without re-verifying them on the hot
+/// path. A hook that contradicts another therefore fails silently in production: a key vanishes from every
+/// enumeration, or a read resolves on the prototype for a property that exists. With the switch on, Jint
+/// recomputes the answer the fast paths exist to avoid and throws on the first disagreement, so these tests
+/// are the checker.
+/// <para>
+/// It has to be set before the first use of any Jint type - the flag is read once at type initialization -
+/// which is what the module initializer guarantees. Never turn it on in production: the verifiers
+/// deliberately redo the work they check.
+/// </para>
+/// </remarks>
+internal static class JintHostContractVerification
+{
+    [ModuleInitializer]
+    internal static void Enable()
+    {
+        AppContext.SetSwitch("Jint.EnableHostContractVerification", true);
+    }
+}