[core] Add file_name and clustering_columns fields to files system table

[heye1005]

Purpose

Closes #7283.

Added file_name and clustering_columns to the $files system table.

• file_name: just the file name, without the path prefix
• clustering_columns: the clustering columns from table schema options, comma-separated. NULL if not configured.

the issue title says "clustering column and file name system table" — not sure if the intent was a standalone $clustering_columns table or just adding fields to an existing one. I went with adding them to $files since it's only two fields and didn't feel like it warranted a separate table. Happy to change if a standalone table is preferred.

Tests

• Updated existing FilesTableTest expected results to include the new fields
• Added testFileNameAndClusteringColumns: creates a table with clustering.columns set, verifies both fields
• Added testClusteringColumnsNull: verifies the field is NULL when clustering is not configured