Unify list-like row decomposition for map construction

## Summary

`datafusion/functions-nested/src/map.rs` has parallel code paths for decomposing list-like key/value inputs when building `MapArray` values. The `List`/`LargeList` path and the `FixedSizeList` path both perform the same conceptual steps: preserve input row count and null-map bitmap, decompose each non-null row into key/value arrays, skip value rows corresponding to null key maps, then call the shared `build_map_array` helper.

This duplication is a maintainability risk. A small shared abstraction for “rows of list-like arrays” would make map construction easier to reason about and reduce the chance that future fixes to null-map handling, offset accounting, or length validation are applied to one list representation but missed in another.

## Current code involved

Relevant functions in `datafusion/functions-nested/src/map.rs` as [at commit](https://github.com/apache/datafusion/tree/96a6096c6f4b924e8cab4bc1629759a948e12939):

- `make_map_array_internal` for `List` and `LargeList`
- `make_map_array_from_fixed_size_list` for `FixedSizeList`
- `list_to_arrays_skipping_null_rows`
- `fixed_size_list_to_arrays_skipping_null_rows`
- `build_map_array`

## Problem

The `List`/`LargeList` and `FixedSizeList` construction paths have nearly identical control flow:

1. Save the original key/value data types.
2. Save the original input row count.
3. Capture the key-array null bitmap, which represents null map rows.
4. Decompose key rows into `Vec`.
5. Decompose value rows while skipping rows where the corresponding key map is null.
6. Delegate to `build_map_array`.

The row-skipping helpers are also duplicated, differing mainly in how they iterate list rows (`as_list::()` vs `as_fixed_size_list()`).

Because these paths encode the same invariants separately, future changes can drift. Areas at risk include:

- preserving null map rows rather than treating them as null keys
- keeping map offsets aligned with original input rows
- handling all-null maps with correct element data types
- validating matching key/value row counts consistently
- maintaining consistent behavior between `List`, `LargeList`, and `FixedSizeList`

## Proposed direction

Introduce a small shared abstraction for list-like row decomposition used by map construction.

Possible shapes:

### Option A: helper taking row iterators

Create one helper that accepts already-decomposed key rows and a value-row iterator/filter strategy, then calls `build_map_array`.

### Option B: enum-backed adapter

Add an internal enum such as `ListLikeArray<'a>` with variants for:

- `List32(&'a ArrayRef)`
- `List64(&'a ArrayRef)`
- `FixedSizeList(&'a ArrayRef)`

Expose methods like:

- `data_type()`
- `len()`
- `nulls()`
- `rows()` / `non_null_rows_skipping(nulls)`

### Option C: small trait/internal helper functions

Use a private trait or generic helper to hide the row iteration difference while keeping the current dispatch from `DataType`.

Any option should keep the public API unchanged and avoid broad architectural changes.

## Suggested implementation constraints

- Keep this refactor local to `datafusion/functions-nested/src/map.rs` unless a clearly reusable utility already belongs elsewhere.
- Preserve current error messages where practical.
- Preserve `List`, `LargeList`, and `FixedSizeList` behavior exactly.
- Do not change SQL-visible semantics.
- Keep `build_map_array` as the single place that constructs offsets, flattened arrays, struct data, and the final `MapArray`, unless the refactor makes an equally clear replacement.

## Test coverage to preserve/add

Existing unit tests around map construction should continue to pass, especially coverage for:

- null map rows
- null key rejection within a map
- `LargeList` input
- `FixedSizeList` input
- mixed scalar/array map arguments

Useful additional targeted tests if gaps remain:

1. `List` and `FixedSizeList` inputs with the same null-row pattern produce equivalent map nulls and offsets.
2. All map rows null still produce an empty child array with the expected key/value element types.
3. Mismatched key/value row counts fail consistently across `List`, `LargeList`, and `FixedSizeList`.

## Acceptance criteria

- The duplicate row-skipping logic for `List`/`LargeList` and `FixedSizeList` is unified or reduced to a thin adapter layer.
- Map construction invariants are documented in one place near the shared helper.
- Existing map unit tests pass.
- Any newly added tests pass with `cargo test -p datafusion-functions-nested --lib map::tests`.
- No public API or SQL behavior changes.

## Scope

This is a refactor only. It is not intended to add new map semantics or change validation behavior.

## Related PR
#22784

Read datafusion/functions-nested/src/map.rs, focusing on make_map_array_internal, make_map_array_from_fixed_size_list, the two row-skipping helpers, and build_map_array. Run cargo test -p datafusion-functions-nested --lib map::tests before changing anything. Done means the duplicate decomposition is unified or reduced to an adapter, invariants remain centralized, and existing behavior and tests are preserved.

Written by the indexing model from the issue text.