[Compute Separation] M2: ConnectedWorkerChannel + channel manager + invocation dispatcher

[brettsam]

New channel type for externally-connected workers. Inherits WorkerChannelBase — no process management. The gRPC stream IS the worker lifecycle.

Dependencies: M1 (#11641)

Deliverables

File	Purpose
ExternalWorkers/ConnectedWorkerChannel.cs	Channel subclass — no IWorkerProcess, no process spawn/terminate
ExternalWorkers/ConnectedWorkerChannelFactory.cs	Creates ConnectedWorkerChannel instances
ExternalWorkers/IConnectedWorkerChannelManager.cs	Interface with AddChannel, GetChannels, WaitForChannelAsync, ShutdownChannelAsync
ExternalWorkers/ConnectedWorkerChannelManager.cs	Implementation with TaskCompletionSource-based WaitForChannelAsync
ExternalWorkers/ConnectedWorkerInvocationDispatcher.cs	IFunctionInvocationDispatcher that routes to connected channels, no restart logic

Acceptance Criteria

• ConnectedWorkerChannel can be constructed and processes the gRPC protocol identically to GrpcWorkerChannel (minus process lifecycle)
• WorkerProcess returns null, WorkerProcessId returns -1
• ConnectedWorkerChannelManager.WaitForChannelAsync() blocks until a channel is added, with configurable timeout
• ConnectedWorkerInvocationDispatcher routes invocations to ready channels
• RestartWorkerAsync returns false (no-op for external workers)
• Unit tests for all new classes