Add a stable server-side MCP Task execution extension point

[cecilphillip]

Is your feature request related to a problem? Please describe.

WithTasks creates a durable task record through IMcpTaskStore, but executes the tool in a process-local Task.Run.

This prevents servers from delegating task execution to durable systems such as Temporal, Orleans, Hangfire, or an external queue. A durable IMcpTaskStore preserves task state, but it cannot control how execution is started.

Replacing the call-tool handler is not a suitable alternative because it relies on experimental alternate-result APIs and replaces parts of the standard MCP tool pipeline.

Describe the solution you'd like

Add a stable extension point that allows WithTasks to delegate execution after the task record has been created.

The extension point should provide:

• The created task information, including task ID.
• The matched tool request and request context.
• Access to the normal tool invocation pipeline when local execution is desired.
• Clear ownership of request-scoped services and disposal.
• Defined ordering for authorization and request filters.
• Cancellation information.

The existing process-local behavior should remain the default.

A custom executor should be able to submit the task to an external durable runtime without invoking the tool body in the MCP server process. Existing tasks/get, tasks/update, and tasks/cancel handling should continue to use IMcpTaskStore.

The exact interface is open for discussion. Conceptually:

public interface IMcpTaskExecutor
{
    ValueTask StartAsync(
        McpTaskExecutionContext context,
        CancellationToken cancellationToken);
}

Describe alternatives you've considered

• Custom IMcpTaskStore: persists state but cannot replace the process-local execution mechanism.
• Custom CallToolWithAlternateHandler: uses experimental APIs and replaces the normal tool pipeline.
• Forking or reimplementing WithTasks: duplicates task negotiation, filtering, DI-scope, cancellation, and result-handling behavior.
• Keeping execution process-local: unsuitable for tasks that must survive server process replacement.

Additional context

The extension point should preserve existing behavior for applications that do not configure a custom executor.

A useful acceptance scenario would be:

1. An MCP task is created.
2. A custom executor submits it to an external durable runtime.
3. The original MCP server process exits.
4. Another server instance continues serving tasks/get, tasks/update, and tasks/cancel.
5. The task reaches a terminal state without depending on the original process.