Enhance client mode handling in ProcessRequest_chat

- Introduced a normalization function for client mode to default to "direct" when input is empty or None.
- Updated the clientMode field to use a new type with BeforeValidator for improved validation.
- Added a test to ensure empty client mode inputs correctly default to "direct".

test/test_client_mode.py

@@ -36,6 +36,18 @@ def test_unknown_client_mode_is_rejected():
         )
 
 
+@pytest.mark.parametrize("client_mode", ["", "  ", None])
+def test_empty_client_mode_defaults_to_direct(client_mode):
+    request = ProcessRequest_chat(
+        sessionId="session-1",
+        timeStamp="1",
+        text="你好",
+        clientMode=client_mode,
+    )
+
+    assert request.clientMode == "direct"
+
+
 @pytest.mark.parametrize(
     ("client_mode", "raw_code", "expected"),
     [