Add 'error_message' as an explicitly required parameter into the FailResult to avoid a footgun.

[JosephCatrambone]

Possible fix for #997

Before:

In [2]: FailResult(message="Heck")
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[2], line 1
----> 1 FailResult(message="Heck")

File ~/.virtualenvs/guardrails/lib/python3.10/site-packages/pydantic/main.py:176, in BaseModel.__init__(self, **data)
    174 # `__tracebackhide__` tells pytest and some other tools to omit this function from tracebacks
    175 __tracebackhide__ = True
--> 176 self.__pydantic_validator__.validate_python(data, self_instance=self)

ValidationError: 1 validation error for FailResult
error_message
  Field required [type=missing, input_value={'message': 'Heck'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.7/v/missing

After:

>>> FailResult("Heck")
FailResult(outcome='fail', error_message='Heck', fix_value=None, error_spans=None, metadata=None, validated_chunk=None)

>>> FailResult(error_message="Heck")
FailResult(outcome='fail', error_message='Heck', fix_value=None, error_spans=None, metadata=None, validated_chunk=None)

>>> FailResult(message="Heck")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 1
----> 1 FailResult(message="Heck")

TypeError: FailResult.__init__() missing 1 required positional argument: 'error_message'

It feels like a bit of a hack to make error_message a required field, since that job really should fall to the BaseModel. This change gives the IDE a type hint (that error_message is the value rather than 'message') and lets people pass a plain string as the first parameter, no named type required.

At some point we should probably shuffle up the stack and remove model_config = ConfigDict(arbitrary_types_allowed=True) from ArbitraryModel, opting for BaseModel where we can.

[dtam]

👍🏼 lgtm