Widen thinking_style from Literal to str | None and add a
@field_validator that produces a helpful error message listing
valid options when an invalid value is provided.
Addresses the review feedback on #4482.
ProviderConfig.thinking_style defaults to None (Optional field), but
create_dynamic_spec expects a string. Coalesce None to "" at all call
sites (factory.py, settings_api.py) and fix the test assertion to
expect None from the config default.