feat: Streaming query support via `IStreamQuery<TResponse>` and `IAsyncEnumerable`

[samtrion]

User Story

As a developer building query-heavy features over large data sets, I want streaming query support via IAsyncEnumerable<TResponse>, so that I can return results incrementally without buffering the entire result set in memory.

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Background

The current IQuery<TResponse> pattern returns a single response object. Scenarios like paginated exports, report generation, or real-time feeds require returning potentially thousands of items. Using IAsyncEnumerable allows callers to process items as they arrive, reducing memory pressure.

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Requirements

• Define IStreamQuery<TResponse> in NetEvolve.Pulse.Extensibility:

  public interface IStreamQuery<TResponse> : IRequest<IAsyncEnumerable<TResponse>> { }

• Define IStreamQueryHandler<TQuery, TResponse> in NetEvolve.Pulse.Extensibility:

  public interface IStreamQueryHandler<TQuery, TResponse>
      where TQuery : IStreamQuery<TResponse>
  {
      IAsyncEnumerable<TResponse> HandleAsync(TQuery query, CancellationToken ct);
  }

• Extend IMediator with:

  IAsyncEnumerable<TResponse> StreamAsync<TQuery, TResponse>(TQuery query, CancellationToken ct = default)
      where TQuery : IStreamQuery<TResponse>;

• Implement StreamAsync in PulseMediator following the same single-handler enforcement as QueryAsync.
• IStreamQueryHandler supports one handler per query type (same as IQueryHandler).
• Interceptors (IQueryInterceptor) do not apply to streaming queries in the initial implementation; streaming queries are executed directly.
• Handler registration follows the existing HandlerRegistrationExtensions patterns.

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Acceptance Criteria

• IStreamQuery<TResponse> and IStreamQueryHandler<TQuery, TResponse> are defined in NetEvolve.Pulse.Extensibility.
• IMediator.StreamAsync is implemented in PulseMediator.
• Registering more than one handler for the same IStreamQueryHandler<TQuery, TResponse> throws InvalidOperationException at dispatch time.
• Dispatching a streaming query with no registered handler throws InvalidOperationException with a descriptive message.
• Cancellation via CancellationToken stops iteration without exception leaking to the caller.
• Unit tests cover: normal streaming, cancellation, missing handler, and duplicate handler scenarios.
• Integration tests verify that a handler returning yield return items is consumed correctly end-to-end.
• XML documentation is provided for all new public members.

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Out of Scope

• Interceptor pipeline support for streaming queries (planned for a follow-up).
• Streaming commands or streaming events.
• Server-Sent Events or SignalR integration.