Allow passing custom stub to execute operations

[andystaples]

Allow passing a custom stub (some implementation of the newly-defined ProtoTaskHubSidecarServiceStub) to the method call operations in the worker.
This is needed for the Azure Functions module, as the protobuf stub will not be available during worker operations - we need to be able to define and pass a custom stub with behavior to capture the inputs to the stub methods used in these methods to redirect back through the Python worker to the functions host indirectly.

[Copilot]

Pull request overview

This PR introduces support for passing custom stub implementations to worker execution methods, enabling Azure Functions to inject custom behavior where a real sidecar stub is unavailable. The key change is the addition of a ProtoTaskHubSidecarServiceStub interface class that can be used as an alternative to the gRPC-generated TaskHubSidecarServiceStub.

• Defines a new ProtoTaskHubSidecarServiceStub class as a protocol for custom stub implementations
• Updates six worker methods to accept either the gRPC stub or the custom stub via Union type annotations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
durabletask/internal/proto_task_hub_sidecar_service_stub.py	Introduces new stub interface class with callable attributes for all TaskHubSidecarService methods
durabletask/worker.py	Updates type annotations for stub parameters in _execute_orchestrator, _cancel_orchestrator, _execute_activity, _cancel_activity, _execute_entity_batch, and _cancel_entity_batch methods to accept custom stub

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[berndverst]

See inline comment.