refactor: extract sort pushdown logic from FileScanConfig into separate module (#21457)

## Which issue does this PR close?

Closes #21433

## Rationale for this change

As noted by @alamb in
#21182 (comment),
`file_scan_config.rs` has grown large after the sort pushdown
optimization. This PR extracts the sort pushdown helpers into their own
module to improve readability and maintainability.

## What changes are included in this PR?

Move sort pushdown logic from `file_scan_config.rs` (3591 → 3066 lines)
into a new `sort_pushdown.rs` module (576 lines):

- `rebuild_with_source`, `try_sort_file_groups_by_statistics`
- `sort_files_within_groups_by_statistics`,
`any_file_has_nulls_in_sort_columns`
- `validate_orderings`, `is_ordering_valid_for_file_groups`
- `get_projected_output_ordering`,
`ordered_column_indices_from_projection`
- `SortedFileGroups` struct

`try_pushdown_sort` stays in the `DataSource` impl — it calls into the
new module.

## Are these changes tested?

Pure refactor, all existing tests pass (120 passed).

## Are there any user-facing changes?

No.

Author: zhuqi-lucas