# Description

The FrameTransformer sensor always assumed that the source frame index
is 0 in the parsed regex expression. However, a recent issue appeared
where this is not the case, and it led to a bug due to the hard-coded
value. This MR fixes the source frame index in the sensor and also adds
a unit test to check this behavior.

Fixes: https://github.com/NVIDIA-Omniverse/Orbit/issues/170

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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Co-authored-by: Mayank Mittal <mittalma@leggedrobotics.com>