feat: add GroupColumn support for Decimal32/Decimal64

[SubhamSinghal]

Which issue does this PR close?

• Part of EPIC: Complete GroupValuesColumn for Faster Aggregation support #24268.

Rationale for this change

Decimal32 and Decimal64 group-by keys currently force the whole grouping onto
the row-encoded GroupValuesRows fallback.

group_column_supported_type is an all-or-nothing gate: if any one column in a
multi-column key is missing from it, new_group_values drops the entire key to
GroupValuesRows, so a (Utf8, Decimal32) key loses the column-wise path for
its string column too:

(Utf8, Decimal32 ) -> GroupValuesRows
(Utf8, Decimal64 ) -> GroupValuesRows
(Utf8, Decimal128) -> GroupValuesColumn

DataFusion has supported these two widths since #17501, and #23849 added
Decimal256 to the allow-list, but the two narrow widths were never backfilled.
Both implement ArrowPrimitiveType, so they reuse the existing
PrimitiveGroupValueBuilder — no new builder is required, exactly like
Decimal128 and Decimal256.

The gap is also internally inconsistent today: Struct("a": Decimal32) is
already supported, because nested types reach the generic RowsGroupColumn
fallback added in #23523. Only the bare top-level type is rejected.

What changes are included in this PR?

• Support Decimal32 / Decimal64 in group_column_supported_type and
  make_group_column.
• Dictionary(K, Decimal32 | Decimal64) starts working as a side effect of the
  existing dictionary recursion in both functions — no extra code.
• Extend the group_column_supported_type_matches_make_group_column
  biconditional test with the two scalar types and the two dictionary-wrapped
  variants.
• Add test_group_values_column_narrow_decimals, which drives both widths
  through one body rather than only the first, asserting supported_schema
  routing, dedup including nulls, and that precision/scale survive emit.
• Add a bench_narrow_decimals benchmark mirroring bench_decimal256.

Are these changes tested?

Yes.

• The consistency test plus the new test_group_values_column_narrow_decimals
  round-trip test in multi_group_by/mod.rs. The round-trip test includes a
  value at each type's full width (999_999_999 and
  999_999_999_999_999_999) so a truncating storage type would fail it.
• Multi-column Decimal32 / Decimal64 GROUP BY with a NULL key in
  group_by.slt. These widths are not reachable from a SQL DECIMAL(p, s)
  declaration — that maps to Decimal128 or Decimal256 by precision — so the
  keys are built with arrow_cast, and each case is paired with an
  arrow_typeof assertion so it cannot silently degrade into more Decimal128
  coverage.

cargo test -p datafusion-physical-plan --lib (1921 passed),
aggregate.slt group_by.slt dictionary.slt decimal.slt, and
dev/rust_lint.sh are all clean.

Are there any user-facing changes?

No. This only changes which GroupValues implementation is selected; grouping
semantics and output types are unchanged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.50746% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.43%. Comparing base (5de9302) to head (993ea78).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/aggregates/group_values/multi_group_by/mod.rs	98.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25470   +/-   ##
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  Coverage   82.43%   82.43%           
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  Files        1140     1140           
  Lines      435862   435929   +67     
  Branches   435862   435929   +67     
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+ Hits       359312   359370   +58     
- Misses      54832    54840    +8     
- Partials    21718    21719    +1     

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[alamb]

Thanks @SubhamSinghal and @martin-g -- i merged up to resolve a conflict to put it in the merge queue