Be stricter about converting rootmodel to dict

`model_to_dict` was recursively converting RootModels, which
could have led to confusing behaviour. We only need to handle
the case of empty input represented by a RootModel, and the
code now raises an error if a RootModel is used otherwise.

Author: rwb27

src/labthings_fastapi/utilities/__init__.py

@@ -4,6 +4,7 @@
 from pydantic import BaseModel, ConfigDict, Field, RootModel, create_model
 from pydantic.dataclasses import dataclass
 from anyio.from_thread import BlockingPortal
+from labthings_fastapi.utilities.introspection import EmptyObject
 
 if TYPE_CHECKING:
     from ..thing import Thing
@@ -63,9 +64,23 @@ def wrap_plain_types_in_rootmodel(model: type) -> type[BaseModel]:
 
 
 def model_to_dict(model: Optional[BaseModel]) -> Dict[str, Any]:
-    """Convert a pydantic model to a dictionary"""
+    """Convert a pydantic model to a dictionary
+
+    We convert only the top level model, i.e. we do not recurse into submodels.
+    This is important to avoid serialising Blob objects in action inputs.
+    This function returns `dict(model)`, with exceptions for the case of `None`
+    (converted to an empty dictionary) and `RootModel`s (checked to see if they
+    correspond to empty input).
+
+    If RootModels with non-empty input are allowed, this function will need to
+    be updated to handle them.
+    """
     if model is None:
         return {}
     if isinstance(model, RootModel):
-        return model_to_dict(model.root)
+        if model.root is None:
+            return {}
+        if isinstance(model.root, EmptyObject):
+            return {}
+        raise ValueError("RootModels with non-empty input are not supported")
     return dict(model)