Fix flaky AsyncContinuation cDAC dump test on osx-arm64 by using Task.Yield (#128004)

> [!NOTE]
> This pull request was prepared with assistance from GitHub Copilot.

## Problem

`AsyncContinuationDumpTests.ThreadLocalContinuation_IsContinuation`
fails intermittently on **osx-arm64 R2R** with:

> Could not find AsyncDispatcherInfo type in CoreLib

Failure tracked in #127774. Recently observed in builds
[1413917](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1413917)
and
[1411121](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1411121).

Fixes #127774

## Root cause

The test relies on the runtime having executed
`AsyncHelpers+RuntimeAsyncTask<T>.DispatchContinuations()`, which writes
`AsyncDispatcherInfo.t_current` and forces the JIT to load the
`AsyncDispatcherInfo` MethodTable.

The current debuggee uses `await Task.Delay(1)` to force a suspension.
On fast machines (in particular Apple Silicon CI machines) the 1ms timer
can fire before the runtime-async `Await` helper checks `IsCompleted`,
so the awaiter is observed as already-completed and the await runs
straight through synchronously. With no suspension there is no dispatch,
no `t_current` write, and the `AsyncDispatcherInfo` MT is never loaded.

I downloaded the failing osx-arm64 dump and walked the crashed thread:

```
[2-4] FailFast
[5]   InnerAsync
[6]   OuterAsync   (async2 body)
[7]   OuterAsync   (thunk)
[8]   Main
[9]   CallEntryPoint
```

There is no `DispatchContinuations` frame on the stack. `OuterAsync`
calls `InnerAsync` directly, confirming the await did not suspend.

I also walked all 3 modules' `TypeDefToMethodTable` maps and confirmed
`AsyncDispatcherInfo` MT is genuinely null everywhere (i.e. cDAC is
reporting reality - the type never got loaded). `Object`, `Task`, and
`Continuation` MTs are all loaded as expected.

## Fix

Replace `await Task.Delay(1)` with `await Task.Yield()` in `InnerAsync`.

`Task.Yield()` returns a `YieldAwaitable` whose awaiter's `IsCompleted`
always returns `false`, so the continuation is unconditionally posted
back via `DispatchContinuations`. This guarantees
`AsyncDispatcherInfo.t_current` is written before `InnerAsync` resumes
and calls `FailFast`, regardless of machine timing.

## Validation

Posting as a draft to run CI across all platforms; the osx-arm64 R2R leg
is the one to watch.

Co-authored-by: Max Charlamb <maxcharlamb@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

src/native/managed/cdac/tests/DumpTests/Debuggees/AsyncContinuation/Program.cs

@@ -18,7 +18,7 @@ internal static class Program
 {
     internal static async Task<int> InnerAsync(int value)
     {
-        await Task.Delay(1);
+        await Task.Yield();
 
         // Crash while still inside Resume — t_current is set on this thread
         // and NextContinuation points to OuterAsync's continuation.