remove RuntimeAsyncTaskCore

[max-charlamb]

Resolves #122355

[dotnet-policy-service]

Tagging subscribers to this area: @mangod9
See info in area-owners.md if you want to be subscribed.

[jkotas]

Is it used from other places - can it be private?

[jkotas]

Alternatively, it may look better for the DispatcherInfo to be a top-level type and have the static pointing the current one contained within the type. Something like:

internal struct AsyncDispatcherInfo
{
....

    [ThreadStatic]
    internal static unsafe AsyncDispatcherInfo* t_current; // Debugger depends on the exact name of this field.
...
}

[Copilot]

Pull request overview

This PR removes the RuntimeAsyncTaskCore nested static class and promotes its contents to the top level, simplifying the code structure. The change refactors how async dispatcher information is organized without altering the functional behavior.

Key changes:

• Moved DispatcherInfo struct from nested RuntimeAsyncTaskCore class to top-level as AsyncDispatcherInfo
• Renamed static field from t_dispatcherInfo to t_current
• Updated all references throughout the DispatchContinuations method to use the new type and variable names