The previous sanitizer proxied `schema.toJsonSchema()`, but fastmcp 3.x uses
`xsschema.toJsonSchema()` which reads `schema["~standard"].jsonSchema.input(...)`
directly when the StandardJSONSchemaV1 extension is present (arktype 2.x).
Our proxy was never invoked, so the sanitizer was a silent no-op.
Proxy the entire `~standard` → `jsonSchema` → `input` chain so the transform
runs regardless of which path xsschema picks. Also add case 1 (add `type:"string"`
to enum-only schemas) — arktype 2.x emits `{enum:["A","B"]}` without a type
field, which is the exact form Gemini rejects with
"only allowed for STRING type".
Verified locally: wrapped schema now emits `{type:"string", enum:[...]}` and
drops `$schema`; validation still works.
Colin McDonnell
6533ffddae