feat: add get_current_state workflow query to BaseWorkflow

Adds a default @workflow.query handler named "get_current_state"
to BaseWorkflow. Returns a _workflow_state string that defaults
to "initialized".

Agents using StateMachine should override this to return the
state machine's current state, enabling external callers to
detect turn completion (state == "waiting_for_input").

Example override:

    @workflow.query(name="get_current_state")
    def get_current_state(self) -> str:
        return self.state_machine.get_current_state()

Companion to scaleapi/scale-agentex#173 which adds
GET /tasks/{task_id}/query/{query_name} endpoint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gabrycina

src/agentex/lib/core/temporal/workflows/workflow.py

@@ -15,6 +15,23 @@ def __init__(
         display_name: str,
     ):
         self.display_name = display_name
+        self._workflow_state: str = "initialized"
+
+    @workflow.query(name="get_current_state")
+    def get_current_state(self) -> str:
+        """Query handler for the current workflow state.
+
+        Subclasses using StateMachine should override this to return
+        the state machine's current state:
+
+            @workflow.query(name="get_current_state")
+            def get_current_state(self) -> str:
+                return self.state_machine.get_current_state()
+
+        This enables external callers to detect when the agent has
+        finished processing a turn (state == "waiting_for_input").
+        """
+        return self._workflow_state
 
     @abstractmethod
     @workflow.signal(name=SignalName.RECEIVE_EVENT)