Make the inner bundle of Parented transparent

[mxgrey]

A small tweak to how to things nested inside of Parented get serialized:

Before

"166": {
    "parent": 1,
    "bundle": {
        "name": "tinyRobot2",
        "pose": {
            "trans": [
                20.423693,
                -5.312098,
                0.0
            ],
            "rot": {
                "yaw": {
                    "deg": 0.0
                }
            }
        },
        "description": 164,
        "optional_properties": []
    }
}

After

"166": {
    "parent": 1,
    "name": "tinyRobot2",
    "pose": {
        "trans": [
            20.423693,
            -5.312098,
            0.0
        ],
        "rot": {
            "yaw": {
                "deg": 0.0
            }
        }
    },
    "description": 164,
    "optional_properties": []
}

This PR reduces some unnecessary struct nesting in the serialized output.

[xiyuoh]

@mxgrey Thank you for taking a look at this! I experimented with it previously and the issue with flattening the Bundle content is that the fields of the bundle will be stored as strings with "". This results in errors when loading the same site file back in. Tested out this PR and here's the error I'm getting when loading a saved file with serde(flatten):

2025-01-20T10:36:12.246615Z ERROR librmf_site_editor::workspace: Failed loading site SpannedError { code: ExpectedIdentifier, position: Position { line: 321, col: 7 } }

Adding a valid format (model_descriptions) vs. the currently error-prone format (model_instances) here for clarity:

model_descriptions: {
    86: (
      name: "OfficeChairBlack",
      source: (Search("OpenRobotics/OfficeChairBlack")),
      is_static: (true),
      optional_properties: ([]),
    ),
  },
  model_instances: {
    87: {
      "parent": 1,
      "name": "OfficeChairBlack",
      "pose": (
        trans: (6.562113, -6.721654, 0.0),
        rot: yaw(deg(243.52425)),
      ),
      "description": Some(86),
      "optional_properties": ([]),
    },

[mxgrey]

Whoops, I hadn't thought of running all the unit tests before opening this PR since I thought it would be a trivial change. Thanks for helping me understand why the RON roundtrip test was failing.

I fell into a bit of a rabbit hole with ron-rs/ron#548 and then ron-rs/ron#555 but it seems everything is working now. For this to work we'll need to target the github repo of RON until its next release, but I think RON will probably have a new release well before we do.

[xiyuoh]

Thanks for hunting down the solution, tried out with the demo map and it works great! ✨ The error remains if we add anything to optional_model_properties, i.e. Tasks which would also require serde(transparent) for loading to work. However these struct are pending some refactor in #263 so I'm happy to give an approval first.

[mxgrey]

@xiyuoh Considering the merge conflicts that will come up due to the changes in optional model properties and the need to add #[serde(transparent)], I'll close this PR and you can just copy/paste the relevant parts from here over to #263.