Silence warning during build.py

[hosackm]

When running build.py, the following warning is issued by pydantic:

$ python build.py
.venv/lib/python3.13/site-packages/pydust/config.py:94: UserWarning: A custom validator is returning a value other than `self`.
Returning anything other than `self` from a top level model validator isn't supported when validating via `__init__`.
See the `model_validator` docs (https://docs.pydantic.dev/latest/concepts/validators/#model-validators) for more details.
  return ToolPydust(**pyproject["tool"].get("pydust", {}))

This warning is issued because there is a missing return statement in pydust/config.py which is interpretted as return None by pydantic. Returning self fixes the issue.

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