feat(physical-plan): add GroupColumn support for FixedSizeList<primitive> in multi-column GROUP BY

PR 2 of the EPIC #22715 sequence. Builds on the dispatcher refactor +
factory framework that landed in PR 1 (#22751) to bring the first
nested type into `GroupValuesColumn`, unblocking the rest of the
sequence (PR 3 Struct, PR 4 List, PR 5 LargeList, PR 6 composite FSL,
PR 7 Map).

## Which issue does this PR close?

Part of #22682 (nested type coverage) and #22715 (EPIC).

## Rationale for this change

Today a single `FixedSizeList<primitive>` column in a GROUP BY drags
the whole grouping onto the byte-encoded `GroupValuesRows` fallback,
even when every other column would have qualified for the column-wise
+ cross-column short-circuit fast path in `GroupValuesColumn`. With
the recursive `make_group_column` factory + `group_column_supported_type`
allow-list in place from PR 1, this PR is now a self-contained
addition of one builder and two dispatcher entries.

## What changes are included in this PR?

- `FixedSizeListGroupValueBuilder<T: ArrowPrimitiveType>` in a new
  `fixed_size_list` submodule. Storage: outer null bitmap + a child
  `PrimitiveGroupValueBuilder<T, true>` that holds every element flat
  (length = `outer_len * list_len`). Element `j` of outer row `i`
  lives at child index `i * list_len + j`.
- `group_column_supported_type` accepts `FixedSizeList<primitive>` for
  the primitive subset wired through the dispatcher (Int8..Int64,
  UInt8..UInt64, Float32, Float64, Date32, Date64). Composite children
  (Struct / List inside FSL) are deferred to a follow-up after their
  respective builders land.
- `make_group_column` dispatches `DataType::FixedSizeList(...)` to the
  appropriate `FixedSizeListGroupValueBuilder<T>` via an
  `instantiate_fsl!` macro mirroring the existing `instantiate_primitive!`
  pattern.

## Are these changes tested?

Yes. 12 new unit tests on `FixedSizeListGroupValueBuilder` plus
extensions to the existing `group_column_supported_type` ⇔
`make_group_column` consistency fuzz.

Per-builder:
- append / build round trip with mixed outer-null and inner-null rows
- `equal_to` for identical / different / outer-null / inner-null rows
- `take_n` boundary cases (`n=0`, `n=len`, prefix containing null rows)
- sliced input array (offset != 0)
- `vectorized_equal_to` / `vectorized_append` match per-row reference
- `size()` grows with appends
- `build` on empty builder returns empty array
- end-to-end dispatcher → `GroupValuesColumn` routing

Consistency fuzz now covers four primitive FSL children (signed,
unsigned, float, date) on the supported side and three non-primitive
FSL children (Utf8, Decimal128, Boolean) on the rejected side, locking
in the scope boundary for this PR.

127/127 aggregates tests pass, clippy clean, fmt clean.

## What follows in the EPIC

- PR 3: `Struct<...>` builder + dispatcher.
- PR 4: `List<T>` builder + dispatcher (recursive child via factory).
- PR 5: `LargeList<T>` builder + dispatcher.
- PR 6: Relax FSL child restriction to allow `FSL<Struct>` / `FSL<List>`
  once the prerequisite child builders are in.
- PR 7: `Map<K,V>` (`List<Struct<keys, values>>` Arrow representation).

Author: zhuqi-lucas