We do not overwrite mismatched migration files in the new naming scheme

We detect when the migration files are out of sync correctly, but I think we didn't update the code once we added the hash to the end of the file name, so we should now delete the old file.

No file or test path is mentioned. Start by locating the migration mismatch detection and the code for the new hash-based filename scheme; verify the mismatched legacy file is deleted and add or update coverage for that behavior.

Written by the indexing model from the issue text.