Function to_github_test_matrix_json 

pub fn to_github_test_matrix_json(entries: &[MatrixEntry]) -> Result<String>

Serialises the matrix to a JSON shape for the test job.

The output includes os so the workflow can use only include entries without relying on matrix cross-product behavior:

{"include": [{"label": "default", "features": "", "os": "ubuntu-latest"}, ...]}

Errors

Returns an error if JSON serialisation fails.