Akka.Streams: add cancellation-aware Source.Queue offers

[orange-dot]

Fixes #8081

Changes

Adds cancellation-aware offer support for Source.Queue while preserving compatibility for existing ISourceQueueWithComplete<T> implementers.

• Adds ICancellableSourceQueueWithComplete<T> with OfferAsync(T element, CancellationToken cancellationToken).
• Adds an ISourceQueueWithComplete<T> extension overload that delegates to cancellable queues and rejects unsupported cancellable calls explicitly.
• Updates QueueSource so pending backpressured offers can be canceled and removed without later emitting the canceled element.
• Adds queue source tests for pending cancellation, already-canceled tokens, CancellationToken.None, completion ordering, no-op cancellation after enqueue, and defensive extension behavior.

API / compatibility

This keeps ISourceQueueWithComplete<T> source-compatible for existing custom implementers. Cancellable support is exposed through the new opt-in ICancellableSourceQueueWithComplete<T> interface and the extension overload requested by #8081.

Validation

• dotnet test src/core/Akka.Streams.Tests/Akka.Streams.Tests.csproj --filter FullyQualifiedName~QueueSourceSpec -c Release --framework net10.0 -v minimal
  • Passed: 37/37
• dotnet test src/core/Akka.API.Tests/Akka.API.Tests.csproj -c Release --framework net10.0 -v minimal
  • Passed: 18/18