Workaround Even Better TOML crash related to `allOf` (#15992)
## Summary
Fixes https://github.com/astral-sh/ruff/issues/15978
Even Better TOML doesn't support `allOf` well. In fact, it just crashes.
This PR works around this limitation by avoid using `allOf` in the
automatically
derived schema for the docstring formatting setting.
### Alternatives
schemars introduces `allOf` whenver it sees a `$ref` alongside other
object properties
because this is no longer valid according to Draft 7. We could replace
the
visitor performing the rewrite but I prefer not to because replacing
`allOf` with `oneOf`
is only valid for objects that don't have any other `oneOf` or `anyOf`
schema.
## Test Plan
https://github.com/user-attachments/assets/25d73b2a-fee1-4ba6-9ffe-869b2c3bc64e
