docs: fix README errors and document generic notification handlers (v2.1.2)

[Copilot]

Several README examples contained incorrect type parameters and a misplaced section heading. Issue #44 also surfaced a missing clarification about IRequest return types when validators/pipelines are involved.

Fixes

• Stream request definition — IStreamRequest<PingDispatchR, ValueTask<int>> → IStreamRequest<CounterStreamRequestDispatchR, int> (element type must not be wrapped in ValueTask; self-reference type was wrong)
• Stream handler definition — handler type was used as the type argument instead of the request type (CounterStreamHandlerDispatchR → CounterStreamRequestDispatchR)
• Notification section heading — "Stream Pipeline DispatchR" → "Notification DispatchR"

Clarification (Issue #44)

Added an [!IMPORTANT] callout under Request Definition: TResponse must always be a generic wrapper (Task<T> / ValueTask<T>). Using bare Task or ValueTask silently breaks handler dispatch when pipeline behaviors or validators are present.

New: Generic Notification Handlers (v2.1.2)

Added docs and example for the open-generic INotificationHandler<TNotification> pattern released in v2.1.2:

public sealed class AllNotificationsLogger<TNotification>(ILogger<AllNotificationsLogger<TNotification>> logger)
    : INotificationHandler<TNotification>
    where TNotification : INotification
{
    public ValueTask Handle(TNotification notification, CancellationToken cancellationToken)
    {
        logger.LogInformation("[Generic] Received notification of type {NotificationType}: {@Notification}",
            typeof(TNotification).Name, notification);
        return ValueTask.CompletedTask;
    }
}

Updated the "Currently supports" list to include this third notification variant.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Documentation-focused update to correct README examples and describe the new open-generic notification handler pattern introduced in v2.1.2, plus a clarification about request return types in DispatchR.

Changes:

• Fix incorrect IStreamRequest<,> and stream handler type parameters in README examples.
• Correct a misplaced notification section heading and add documentation/example for open-generic INotificationHandler<TNotification>.
• Add an [!IMPORTANT] callout about request TResponse return types when pipelines/validators are involved.

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