Updated get_dataset_config_names returning default in offline mode

[abigailllr]

When a dataset is cached and accessed in offline mode, get_dataset_config_names was returning default instead of the actual cached config names. This happened because CachedDatasetModuleFactory.get_module returned a DatasetModule without builder_configs_parameters, causing the fallback to default in get_dataset_config_names.

The fix reads config_name from each dataset_info file in the cache directory and includes them as builder_configs_parameters in the returned DatasetModule. Invalid or missing dataset_info.json files are handled.

Testing:

1. Download a dataset in online mode so it gets cached
2. Switch to offline mode and call get_dataset_config_names
3. Verify it returns the cached config names instead of ['default']

Example:

• HF_DATASETS_OFFLINE=0 HF_HOME="/tmp/hftemp" python -c "import datasets; datasets.load_dataset('cais/mmlu', 'all')"
• HF_DATASETS_OFFLINE=1 HF_HOME="/tmp/hftemp" python -c "import datasets; print(datasets.get_dataset_config_names('cais/mmlu'))"
• -> Expected output: ['all']

Fixes #7947