Support for ManualCompletion in TestActivity Env

Is your feature request related to a problem? Please describe.

Testing an Activity in isolation is feasible with the TestActivityEnvironment. However is a challenge when using ManualActivityCompletionClient

var manualActivityCompletionClient =
          Activity.getExecutionContext().useLocalManualCompletion();

1. Error: Method temporal.api.workflowservice.v1.WorkflowService/RespondActivityTaskCompleted is unimplemented'
2. Alternatively, I still couldn't test it with a Test Interceptor based approach. Down the useLocalManualCompletion implementation, it returns new clients, so unable to mock.

Describe the solution you'd like
Either

• support to test with ManualCompletionClient (or)
• ability to return a mock of ManualActivityCompletionClient in useLocalManualCompletion(). Maybe a client supplier? (or)

Describe alternatives you've considered

• tried interceptor approach to test and with TestEnvironment

Additional context

• or should we discourage usage of ManualCompletionClient in java docs .

Start with TestActivityEnvironment and the Activity.getExecutionContext().useLocalManualCompletion() path, then compare it with the Test Interceptor and TestEnvironment approaches described in the issue. Determine whether the intended outcome is ManualActivityCompletionClient support in tests or a mockable client supplier, and define a test that demonstrates the chosen behavior.