Remove undefined-behavior UnsafeTests.DangerousAs to stabilize jitstress leg

[Copilot]

This change addresses an intermittent failure in System.Runtime.CompilerServices.UnsafeTests.DangerousAs on jitstress ARM runs. The test asserted behavior from an invalid cast path (Unsafe.As<string>(object)) where the result is undefined.

Fixes#129678

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[Copilot]

Pull request overview

This PR removes a unit test that exercises Unsafe.As<string>(object) on an incompatible runtime type, which can produce runtime/JIT-dependent behavior and intermittently fail under stress configurations (e.g., ARM jitstress). The remaining As() test still validates the supported/meaningful usage (object containing a string cast to string).

Changes:

• Deleted UnsafeTests.DangerousAs (invalid cast scenario) from UnsafeTests.cs.
• Left the existing As() test intact.

[MichalPetryka]

This change addresses an intermittent failure in System.Runtime.CompilerServices.UnsafeTests.DangerousAs on jitstress ARM runs.

Do the other invalid usages in runtime like Unsafe.As<RawArrayData>(array) need to be fixed for this too then?

[jkotas]

Do the other invalid usages in runtime like Unsafe.As(array) need to be fixed for this too then?

These internal runtime uses in CoreLib are in patterns that do not cause problems in the JIT currently. If they start causing problems, they will have to be fixed.