## Problem
When users configure auto-metadata for a dataset, parsing crashes with:
```
KeyError: 'properties' in gen_metadata → schema["properties"]
```
## Root Cause
Pydantic `AutoMetadataField` defaults `enum` and `description` to `None`
when the frontend omits these fields:
```python
class AutoMetadataField(Base):
enum: Annotated[list[str] | None, Field(default=None)]
description: Annotated[str | None, Field(default=None)]
```
These `None` values propagate through the call chain and cause two
crashes:
3eff41361b