[Compute Separation] M3: Inbound connection acceptance in FunctionRpcService

[brettsam]

Modify FunctionRpcService.EventStream to accept unknown workerIds when external worker mode is enabled. Today, unknown worker IDs are silently ignored. We add an else branch.

Dependencies: M2 (#11643), PROTO (#11642)

Deliverables

File	Action
Server/FunctionRpcService.cs	Add inbound branch for unknown worker IDs; inject new dependencies
ExternalWorkers/WorkerConnectionService.cs	IHostedService — responds to inbound connections, creates channels
ExternalWorkers/WorkerConnectedEvent.cs	Event published when connected worker completes init handshake

Acceptance Criteria

• External worker can connect to gRPC endpoint with an unknown workerId
• WorkerConnectionService creates ConnectedWorkerChannel and completes init handshake
• WorkerConnectedEvent published after WorkerInitResponse received
• Existing host-managed worker path (TryGetGrpcChannels succeeds) is unmodified
• Optional constructor params keep existing DI valid when feature disabled
• Unit tests