feat(mcp): add in process work item stream for internal mcp wf routing

[sicoyle]

Dapr's MCP Server integration registers built-in dapr.mcp.* orchestrations and dapr-mcp-* activities that run inside the sidecar. Initially I tried using a parallel task.TaskExecutor behind a RoutingExecutor wrapper, but that ended up being a completely separate execution path from the normal gRPC-backed engine. That meant that at first, internal MCP workflows couldn't use the standard TaskHubWorker lifecycle, queue-backed parallelism, or backend-driven retries.

With this PR, Dapr can register an InProcessClient as a sink for dapr.mcp.* prefixed names. The gRPC executor routes matching work items to the sink while everything else flows to the external SDK unchanged. One engine, one backend, two delivery paths - internal workflows are authored identically to SDK workflows.

CHanges:

• Adds WorkItemSink / SinkRegistrar interfaces on grpcExecutor to route work items to in-process consumers by workflow-name and activity-name prefix, bypassing the gRPC stream
• Adds client.InProcessClient — an in-process WorkItemSink implementation that mirrors TaskHubGrpcClient using a shared dispatcher.go for execute+response logic
• Extracts shared dispatchWorkflow / dispatchActivity helpers from the duplicated code in worker_grpc.go and worker_inprocess.go
• Sinks are closed before the gRPC work-item queue during Shutdown

[Copilot]

Pull request overview

This PR adds an in-process work-item routing path to the existing gRPC-backed executor so internal dapr.mcp.* workflows/activities can be delivered to in-process consumers while all other work items continue to flow over the external SDK gRPC stream.

Changes:

• Introduces backend.WorkItemSink and backend.SinkRegistrar to register sinks by workflow/activity name prefix and route matching work items in-process.
• Adds client.InProcessClient as an in-process sink implementation and factors shared workflow/activity dispatch logic into client/dispatcher.go.
• Adds tests validating sink registration, routing behavior, and shutdown sink closing.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
backend/executor.go	Adds sink registration APIs, prefix matching, routing logic in ExecuteWorkflow/ExecuteActivity, and closes sinks first during Shutdown.
client/worker_inprocess.go	Adds an in-process worker/sink that drains work items from an internal channel and completes tasks via the backend.
client/worker_grpc.go	Refactors workflow/activity processing to use shared dispatch helpers.
client/dispatcher.go	Centralizes workflow/activity execution + response construction shared by gRPC and in-process workers.
tests/sink_test.go	Adds unit tests covering sink registrar validation, routing, and shutdown close behavior.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

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