Fix metadata export when config rows are non-consecutive

[KirtiRamchandani]

Fixes #8269

groupby() on unsorted exported parquet rows dropped earlier shards when the same config reappeared later in the list. Sort by (config, split) first.

Regression test: test_non_consecutive_config_rows_are_merged_in_metadata_configs_from_exported_parquet_files

rootdir: /tmp/datasets
configfile: pyproject.toml
plugins: datadir-1.8.0, xdist-3.8.0, anyio-4.13.0, hypothesis-6.155.2
collecting ... collected 1 item

tests/test_metadata_util.py::test_non_consecutive_config_rows_are_merged_in_metadata_configs_from_exported_parquet_files PASSED

============================== 1 passed in 0.39s ===============================