IAsyncEnumerable: Fix cancellation propagation, bound operation times, use mark-and-sweep cleanup

[ReubenBond]

This PR improves the implementation of IAsyncEnumerable<T> support.

1. Canceled tasks are propagated to callers.
2. All asynchronous operations (MoveNextAsync, cancellation, disposal) have bounded times, based on MessagingOptions.ResponseTimeout.
3. Cleanup is reimplemented based on a mark-and-sweep algorithm which won't clean up enumerators which are actively in-use.
4. Test coverage has been improved.

Microsoft Reviewers: Open in CodeFlow

Pull Request Overview

This PR refactors the IAsyncEnumerable support to propagate cancellation, bound asynchronous operation times, and improve enumerator cleanup via a mark‐and‐sweep algorithm while also adding new tests.

• Introduces new tests to cover cancellation, slow consumer, and eviction scenarios
• Refactors AsyncEnumerableGrainExtension to use a mark‐and‐sweep cleanup and bounds on asynchronous operations
• Updates MessagingOptions and AsyncEnumerableRequest to support new timeout configurations

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/DefaultCluster.Tests/AsyncEnumerableGrainCallTests.cs	Added new test cases for cancellation and slow consumer scenarios
src/Orleans.Core/Runtime/AsyncEnumerableGrainExtension.cs	Refactored internals for cancellation propagation and cleanup, including changes to task handling and flag management
test/Grains/TestInternalGrains/ObservableGrain.cs	Minor reordering in error handling for GetValuesWithError
src/Orleans.Core.Abstractions/Runtime/AsyncEnumerableRequest.cs	Updates to incorporate WaitAsync and cancellation handling
src/Orleans.Core/Configuration/Options/MessagingOptions.cs	Added a new internal property to expose the configured response timeout
test/Grains/TestGrainInterfaces/IObservableGrain.cs	Minor namespace whitespace update

Comments suppressed due to low confidence (1)

src/Orleans.Core/Runtime/AsyncEnumerableGrainExtension.cs:22

• The initialization of '_enumerators' uses '[]', but a new Dictionary should be instantiated using 'new Dictionary<Guid, EnumeratorState>()' to ensure proper behavior.

private readonly Dictionary<Guid, EnumeratorState> _enumerators = [];