An activity call to a non-existent function should error-out

Currently, if we try to make an activity call to a non-existent function, the orchestrator simply sleeps indefinitely. We should instead validate that the function exists and error out if not.

Additionally, we should consider doing some basic typo-checking in case the function exists under a slightly different name. Think of `Hello` versus `Hello1`. The latter is a scenario that you can get into when using the plugin to create functions.

Start by tracing the orchestrator's activity-call handling and reproduce a call to a non-existent function to identify where it remains pending. Done means the call errors instead of sleeping indefinitely; investigate whether a nearby-name diagnostic such as Hello versus Hello1 can also be specified and tested.

Written by the indexing model from the issue text.