fix(gateway): make WriteEngine + converter raise-safe against malformed write commands

ingestor/converters.py

@@ -25,6 +25,8 @@ def to_typed_value(raw, data_type: str):
     data_type: 'boolean' | 'int' | 'float' | 'double' | 'string'.
     Raises ConversionError on any invalid input (never a bare TypeError).
     """
+    if not isinstance(data_type, str):
+        raise ConversionError(f'data_type must be a string, got {type(data_type).__name__}')
     dtype = data_type.strip().lower()
     try:
         if dtype in ('bool', 'boolean'):