Remove Rx dependencies

[Eli Arbel (aelij)]

Updated deprecated packages:

• Removed System.Reactive.* - only a simple Subscribe extension method was used and can be embedded in the repo.
• Replaced System.Linq.Async with System.Linq.AsyncEnumerable.

[Copilot]

Pull request overview

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This PR removes the System.Reactive dependency and replaces prior “Subscribe(Action)” usages with a lightweight local observer implementation, while also updating async-enumerable-related dependencies.

Changes:

• Removed System.Reactive.* package references from DurableTask.Core and replaced Rx-based Subscribe usage with a custom ActionObserver<T>.
• Swapped System.Linq.Async for System.Linq.AsyncEnumerable in AzureStorage and updated the ToDictionaryAsync callsite to use a named cancellationToken argument.
• Updated centralized package versions for async interfaces and async-enumerable packages.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/DurableTask.Core/DurableTask.Core.csproj	Drops System.Reactive package references to remove Rx dependency.
src/DurableTask.Core/CorrelationTraceClient.cs	Replaces Rx Subscribe(Action<T>) with custom IObserver<T> implementation.
src/DurableTask.AzureStorage/OrchestrationSessionManager.cs	Adjusts ToDictionaryAsync invocation for updated async-enumerable package API.
src/DurableTask.AzureStorage/DurableTask.AzureStorage.csproj	Replaces System.Linq.Async with System.Linq.AsyncEnumerable.
Directory.Packages.props	Updates package versions to match new dependency set.

Comments suppressed due to low confidence (1)

src/DurableTask.Core/CorrelationTraceClient.cs:1

• The nested new ActionObserver<...>(...) allocations add verbosity and make the subscription logic harder to scan. Consider factoring a small helper (e.g., a private static Observer(Action<T> onNext) factory) so the subscription reads closer to the original intent, and so any future changes to error/completion behavior are centralized in one place.

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[Eli Arbel (aelij)]

That's incorrect

https://github.com/dotnet/reactive/blob/56b3e1e9bf3605a4e2bfe08b68c11291896363ab/Rx.NET/Source/src/System.Reactive/Observable.Extensions.cs#L63