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because this is no longer valid according to Draft 7.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

ruff.schema.json

@@ -879,7 +879,7 @@
       "anyOf": [
         {
           "description": "Wrap docstring code examples at a fixed line width.",
-          "allOf": [
+          "oneOf": [
             {
               "$ref": "#/definitions/LineWidth"
             }
@@ -887,18 +887,10 @@
         },
         {
           "description": "Respect the line length limit setting for the surrounding Python code.",
-          "allOf": [
-            {
-              "$ref": "#/definitions/Dynamic"
-            }
-          ]
+          "const": "dynamic"
         }
       ]
     },
-    "Dynamic": {
-      "type": "string",
-      "const": "dynamic"
-    },
     "Flake8AnnotationsOptions": {
       "description": "Options for the `flake8-annotations` plugin.",
       "type": "object",