fix: scan all-null columns after dropping physical columns

[skyshineb]

Summary

Closes #7236
Fix scans after dropping the only physical column from a dataset that still has row-bearing all-null schema-evolution columns.

Operation::Project can leave fragments with files = [] while retaining physical_rows. Scans need that row count to synthesize null arrays for the remaining projected columns.

Changes

• Trust modern Fragment.physical_rows before rejecting file-less fragments in FileFragment::physical_rows().
• Update fragment validation to allow file-less fragments when physical_rows is present.
• Use physical_rows for deletion-vector bounds in file-less projected fragments.
• Add regressions for scanning all-null columns and appending after re-adding a dropped physical column.