Handle expired Keycloak session with user-friendly error message (#12168)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/server/types.py

@@ -50,3 +50,9 @@ class LLMAuthenticationError(ValueError):
     """Raised when there is an issue with LLM authentication."""
 
     pass
+
+
+class SessionExpiredError(ValueError):
+    """Raised when the user's authentication session has expired."""
+
+    pass