Add runtime async tests for catching non-Exception throws

[Copilot]

Runtime async (async2) is meant to match compiler state-machine async (async1). No coverage existed for catching objects that don't derive from System.Exception (only throwable via IL), where RuntimeCompatibilityAttribute(WrapNonExceptionThrows) governs whether the object surfaces as a RuntimeWrappedException. These tests close that hole and pin down where the two forms agree and diverge.

Description

New tests under src/tests/async/runtime-wrapped-exception/:

• NonExceptionThrower.il — IL helper that throws a non-Exception object (a string), since C# can't express this.
• NoWrapThrowers.cs — helper assembly marked [assembly: RuntimeCompatibility(WrapNonExceptionThrows = false)] with async1/async2 throwers. The attribute is assembly-scoped, so an opt-out variant requires its own assembly.
• runtime-wrapped-exception.cs — awaits each thrower (async1 via [RuntimeAsyncMethodGeneration(false)], async2 by default) and asserts the caller observes a RuntimeWrappedException wrapping the thrown object, for throws both before and after a suspension point, under both WrapNonExceptionThrows settings.

Observed behavior:

• WrapNonExceptionThrows = true (C#/CoreLib default): async1 and async2 match — caller sees RuntimeWrappedException in all cases.
• WrapNonExceptionThrows = false: they match for a synchronous (pre-yield) throw and for async2 after a yield. They diverge for a non-Exception thrown after a suspension point — async2 faults the Task with a RuntimeWrappedException, while async1 lets the raw object escape onto the thread pool and crash the process.

The divergent (crashing) case, CatchAfterYield_NoWrap_Async1, is not exercised as a test: this reflects the older async1 behavior that is terminal to the process and is not something we plan to change, so it is left blocked with an explanatory comment rather than an [ActiveIssue] link. The remaining tests run.

On NativeAOT a non-Exception thrown after a suspension point is not surfaced as a RuntimeWrappedException and terminates the process instead of faulting the Task, so the test project is blocked on NativeAOT (DisableProjectBuild when TestBuildMode == nativeaot), pending the follow-up #131111.

// async1 (state machine) vs async2 (runtime async) — same body, different generation
public static async Task ThrowAfterYieldAsync2()
{
    await Task.Yield();
    NonExceptionThrower.ThrowNonException(); // throws a string (IL)
}

[RuntimeAsyncMethodGeneration(false)]
public static async Task ThrowAfterYieldAsync1()
{
    await Task.Yield();
    NonExceptionThrower.ThrowNonException();
}

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