[Bug]: BoundingBoxes2D: describe the existing artifact JSON list in annotations #12787

Describe the bug

wandb.data_types.BoundingBoxes2D.to_json is annotated to return dict, but its artifact serialization deliberately returns a list of box dictionaries. The paired from_json advertises json_obj: dict while requiring that list representation.

The list format is intentional for compatibility: current source and comment. The request is to align the annotations with that existing format, including the different Run/Artifact branches.

Minimal reproduction

import wandb
from wandb.data_types import BoundingBoxes2D

data = [{
    "position": {"minX": 0.1, "maxX": 0.2, "minY": 0.3, "maxY": 0.4},
    "class_id": 0,
}]
boxes = BoundingBoxes2D({"box_data": data}, key="predictions")
artifact = wandb.Artifact("example", type="dataset")
payload = boxes.to_json(artifact)
print(type(payload).__name__)  # list, despite -> dict
assert BoundingBoxes2D.from_json(payload, artifact).to_json(artifact) == payload
BoundingBoxes2D.from_json({"box_data": data}, artifact)

The actual list round-trip succeeds. The dictionary call fails:

TypeError: Boxes must be a list

This uses a real local Artifact and valid box coordinates; no run or server is involved. Image.from_json also consumes the list representation through this helper.

Expected behavior

Make the serializer/deserializer annotations describe the supported payloads without changing their existing serialization format.

Environment

• W&B SDK: 0.30.0; relevant source is identical on main 8f93d444e29e4d1a50a888899f51051012ee2679
• Python: 3.12.12
• OS: macOS 26.1, arm64
• W&B server: not applicable