Call an entity from an orchestration

[ahmedmuhsin]

Part of #141

Goal

Let an orchestrator call an entity operation and await its result from task.OrchestrationContext.

Scope

• Add a call entity method on the orchestration context that sends an operation request to a target EntityInstanceId and waits for the OperationResult.
• Surface the returned value, and turn an operation failure into an error the orchestrator can handle.
• Emit the entity request as an orchestrator action so the backend routes it to the entity.

Reference

durabletask-dotnet TaskOrchestrationEntityFeature.CallEntityAsync and its typed result form. The call is modeled as an operation message plus an external event carrying the OperationResult.

Depends on

The foundational entity runtime, and the backend entity scheduler it introduces.