fix(transform schema): preserve $defs when schema root is a $ref (#1642)

transform_schema() early-returns when the schema root contains $ref,
which discards a sibling $defs and produces a schema with a dangling
reference. Pydantic v2 emits exactly this shape for RootModel types
({"$ref": "#/$defs/X", "$defs": {...}}), so any RootModel output
type hits this.

Process $defs before the $ref early-return so referenced definitions
are preserved. Bare $ref schemas (no siblings) are unchanged.