Enable `_concat_source_group` single file pass-through to avoid rewrites

If there is only one file from earlier data work, add logic to `cytotable.convert._concat_source_group` to avoid rewriting an individual file for concatenation operations.

Read cytotable.convert._concat_source_group and trace how it handles concatenation when the source group contains one file. Confirm the existing behavior with the relevant conversion workflow, then verify that a single input is passed through without being rewritten while multi-file concatenation remains unchanged.

Written by the indexing model from the issue text.