Address runtime async TaskScheduler TODO

[jakobbotsch]

Avoid an unnecessary allocation for the TaskScheduler case.

[Copilot]

Pull request overview

This PR refactors the non-default TaskScheduler await-continuation path so CoreCLR’s runtime async continuation code can schedule/invoke the continuation without allocating a TaskSchedulerAwaitTaskContinuation instance, while preserving the existing inline-vs-queue behavior.

Changes:

• Extracts the “inline or schedule onto non-default TaskScheduler” logic into TaskSchedulerAwaitTaskContinuation.RunOrScheduleAction(...).
• Refactors AwaitTaskContinuation.CreateTask(...) to a protected static helper that takes an explicit ExecutionContext? argument.
• Updates CoreCLR runtime async continuation sites to call the new helper directly (removing the prior TODO and per-continuation object allocation).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskContinuation.cs	Adds RunOrScheduleAction helper and refactors task creation to avoid needing an instance for the TaskScheduler path.
src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeAsyncTaskContinuation.cs	Uses RunOrScheduleAction directly instead of allocating TaskSchedulerAwaitTaskContinuation.
src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs	Uses RunOrScheduleAction directly in the captured-TaskScheduler continuation follow-up path.