Initiate migration before IGrainManagementExtension.MigrateOnIdle completes

[ReubenBond]

This PR attempts to fix test flakiness (and flakiness observed by callers) when remotely calling IGrainManagementExtension.MigrateOnIdle() on a grain by ensuring that migration has been initiated (and the activation is no longer valid) before the call completes.

This does not guarantee that a this.MigrateOnIdle() call from within a grain will prevent the grain from processing any future requests (i.e, it could still remain valid for a short period).

It also includes a drive-by fix for the timing-sensitive ErrorHandlingTimedMethodWithError test.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR modernizes asynchronous code patterns by replacing blocking Thread.Sleep calls with proper async/await using Task.Delay, and improves the migration workflow to support async completion signaling.

• Converted synchronous blocking methods to proper async/await patterns in ErrorGrain
• Simplified test timing logic by removing complex stopwatch-based race conditions
• Enhanced migration workflow to properly signal completion using TaskCompletionSource

Reviewed Changes

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

File	Description
test/Grains/TestInternalGrains/ErrorGrain.cs	Converted LongMethod and LongMethodWithError from blocking Thread.Sleep to async Task.Delay; fixed UnobservedErrorDelayed to properly use async lambda with Task.Factory.StartNew
test/DefaultCluster.Tests/ErrorGrainTest.cs	Simplified ErrorHandlingTimedMethodWithError test by removing timing-dependent assertions that caused race conditions
src/Orleans.Runtime/Catalog/ActivationData.cs	Refactored migration workflow to support async completion signaling via TaskCompletionSource in MigrateOnIdle; changed DeactivateCore to private visibility