block drivers from editing trip departure_date

- remove departure_date from update_trip_field enum
- remove departure_date handling in _build_trip_field_update
- update tool descriptions to warn model date is immutable

app/ai/tool_schemas.py

@@ -275,7 +275,7 @@ _UPDATE_TRIP_FIELD = {
     "type": "function",
     "function": {
         "name": "update_trip_field",
-        "description": "Update one field on the trip the driver is editing. trip_id from active session.",
+        "description": "Update one field on the trip the driver is editing. trip_id from active session. Note: departure_date cannot be changed once the trip is created.",
         "parameters": {
             "type": "object",
             "properties": {
@@ -284,7 +284,6 @@ _UPDATE_TRIP_FIELD = {
                     "enum": [
                         "departure",
                         "destination",
-                        "departure_date",
                         "departure_time",
                         "pickup_time",
                         "vehicle_type",
@@ -292,11 +291,11 @@ _UPDATE_TRIP_FIELD = {
                         "total_seats",
                         "price",
                     ],
-                    "description": "Trip field to update.",
+                    "description": "Trip field to update. departure_date is not allowed.",
                 },
                 "value": {
                     "type": "string",
-                    "description": "New value. Dates YYYY-MM-DD, time HH:MM, routes Arabic, seats/price digits.",
+                    "description": "New value. time HH:MM, routes Arabic, seats/price digits. Do NOT include date — departure_date cannot be changed.",
                 },
             },
             "required": ["field", "value"],

app/tools/handlers.py

@@ -772,12 +772,6 @@ class FalsaToolHandlers:
         if field in {"departure", "destination"}:
             return {field: value}, None
 
-        if field == "departure_date":
-            parsed_date = _parse_date_value(value)
-            if not parsed_date:
-                return {}, "departure_date must be YYYY-MM-DD"
-            return {"departure_date": parsed_date.isoformat()}, None
-
         if field in {"departure_time", "pickup_time"}:
             bucket = normalize_departure_bucket(value)
             if not bucket: