Preserve orchestration context compatibility

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 42bcd2ff-9f6f-4235-81b5-629d07ecaa30

Author: andystaples

.github/copilot-instructions.md

@@ -48,6 +48,17 @@ Examples:
 - Follow PEP 8 conventions.
 - Use `autopep8` for Python formatting.
 
+## Public API Compatibility
+
+- Treat adding an abstract method or property to a public ABC as a breaking
+  change because existing third-party subclasses will fail at instantiation.
+- Do not add new abstract members outside a major release. For additive APIs in
+  non-major releases, provide a concrete default implementation that preserves
+  existing subclass instantiation and raises `NotImplementedError` only when
+  the new API is invoked.
+- Add a regression test that confirms a new concrete API is absent from the
+  public ABC's `__abstractmethods__`.
+
 ## Copyright Headers
 
 Every new Python (`.py`) source file MUST begin with the following copyright

durabletask/task.py

@@ -362,7 +362,6 @@ def wait_for_external_event(self, name: str, *,
         """
         pass
 
-    @abstractmethod
     def send_event(self, instance_id: str, event_name: str, *,
                    data: Any | None = None) -> None:
         """Send an event to another orchestration instance.
@@ -381,7 +380,10 @@ def send_event(self, instance_id: str, event_name: str, *,
         data : Any | None
             The optional serializable event payload.
         """
-        pass
+        raise NotImplementedError(
+            "This OrchestrationContext implementation does not support "
+            "send_event()."
+        )
 
     @abstractmethod
     def continue_as_new(self, new_input: Any, *, save_events: bool = False) -> None:

tests/durabletask/test_orchestration_executor.py

@@ -1426,6 +1426,8 @@ def orchestrator(ctx: task.OrchestrationContext, _):
 
 def test_send_event_action():
     """An orchestration can emit a one-way event action."""
+    assert "send_event" not in task.OrchestrationContext.__abstractmethods__
+
     payload = {"approved": True, "approver": "Ada"}
 
     def orchestrator(ctx: task.OrchestrationContext, _):