Refactor system message handling to use event stream (#7824)

Co-authored-by: openhands <openhands@all-hands.dev>
Co-authored-by: Calvin Smith <email@cjsmith.io>

openhands/core/schema/action.py

@@ -6,6 +6,10 @@ class ActionType(str, Enum):
     """Represents a message.
     """
 
+    SYSTEM = 'system'
+    """Represents a system message.
+    """
+
     START = 'start'
     """Starts a new development task OR send chat from the user. Only sent by the client.
     """