fix(tool_transform): hoist $defs to schema root when ArgTransform introduces them - #4101

TransformedTool._apply_single_transform calls get_cached_typeadapter(transform.type).json_schema() and merges the result into the per-property new_schema with new_schema.update(type_schema). TypeAdapter.json_schema() returns a self-contained schema with $defs at its top level, so the update plants those defs inside the property node while the $ref values still point at #/$defs/... (the schema root). Any consumer that resolves refs raises KeyError.

_create_forwarding_transform already hoists the parent tool's pre-existing $defs, but when the parent declares the parameter as a plain type (e.g. dict | None), parent_defs is empty and the hoist branch never fires. _apply_single_transform now pops $defs off the TypeAdapter schema before merging into the property schema and returns them alongside; the caller folds them into parent_defs so they land at the schema root via the same path that already handles parent defs (including compress_schema's unused-def pruning).

class Filter(BaseModel):
    field: str
    op: str

@base.tool
def search(filters: dict | None = None) -> list[str]: ...

base.add_tool_transform(
    "search",
    arg_transforms={"filters": ArgTransform(type=list[Filter])},
)
# Before: $defs lands inside the property node; refs to #/$defs/Filter resolve to nothing.
# After:  $defs hoists to the schema root; refs resolve cleanly.

Closes #4093.

• If I used an LLM, it followed the repo's contributing conventions (not generic output)