fix(config): report computed fields as read-only on update (#1454)

PUT /v1/config/{path} answered "does not exist in model" when the
target was a computed field such as eos_device_instruction_entity_ids.
Raise an explicit read-only error instead, so API clients can tell
read-only fields from unknown ones.

Closes #913

src/akkudoktoreos/core/pydantic.py

@@ -569,6 +569,11 @@ class PydanticModelNestedValueMixin:
             raise TypeError(f"Model '{model}' is not of class type.")
 
         if key not in model.model_fields:
+            if key in model.model_computed_fields:
+                raise TypeError(
+                    f"Field '{key}' of model '{model.__name__}' is a read-only computed "
+                    "field and cannot be updated."
+                )
             raise TypeError(f"Field '{key}' does not exist in model '{model.__name__}'.")
 
         field_annotation = model.model_fields[key].annotation