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

[zhuqi-lucas]

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

Are there any user-facing changes?

No behavior change for users whose GROUP BY did not previously contain a `FixedSizeList` column. For users who did, the grouping now uses the column-native fast path instead of falling back to `GroupValuesRows` — same results, less memory and CPU.

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