fix(datafusion): handle coalesced multi-file batches in next-scan (delta-io#4112)

# Description

Fix next-scan execution when upstream coalescing produces batches with
rows from multiple files.

Changes:
- Split incoming batches into contiguous file_id runs before applying DV
masks/transforms
- Buffer fan-out outputs via VecDeque to preserve row order
- Return `internal_datafusion_err!` on unexpected file_id column type
instead of panicking
- Add tests for interleaved file IDs, fanout, and invalid/null file_id
paths

# Related Issue(s)
<!---
For example:

- closes #106
--->

# Documentation

<!---
Share links to useful documentation
--->

Signed-off-by: Ethan Urbanski <ethan@urbanskitech.com>

Author: ethan-tyler