gz sdf --inertial-stats should process world files

## Desired behavior

Currently the `gz sdf --inertial-stats` command can parse SDFormat files containing a single model and print aggregate inertial statistics (total mass, aggregate center of mass and moment of inertia matrix), but it fails when trying to load an SDFormat world file with `Error: Expected a model file but received a world file.`

It would be convenient to parse a world file and print the inertial stats for each model contained in the world along with the name of the model.

## Alternatives considered

## Implementation suggestion

## Additional context

Start by locating the implementation and tests for `gz sdf --inertial-stats`, then inspect how world files and their models are parsed. Done means the command accepts a world file and prints each model's name with its aggregate inertial statistics, while preserving existing single-model behavior.

Written by the indexing model from the issue text.