minor: derive group_column_supported_type from make_group_column

[jayzhan211]

Which issue does this PR close?

• N/A — small refactor, no issue filed.

Rationale for this change

multi_group_by keeps the set of types GroupValuesColumn supports in two places:

• make_group_column, the factory that builds a GroupColumn for a field, and
• group_column_supported_type, a hand-written allow-list that supported_schema uses to choose between GroupValuesColumn and the GroupValuesRows fallback.

The two must accept exactly the same types: if the allow-list accepts a type the factory rejects, the planner picks GroupValuesColumn and the query then fails with not_impl_err when the columns are built. Today that is enforced by "keep the two in lockstep" comments on both sides (valid Time32/Time64 units, non-negative FixedSizeBinary width, the nested/row-backed fallback, recursion into List and Dictionary) and by a pinning test over a sample of types. Every new specialization has to be added twice.

One case had already drifted: a Dictionary with a non-integer key type is accepted by the allow-list but rejected by the factory. It is not a valid Arrow type, so it is not reachable in practice, but it shows the shape of the problem.

What changes are included in this PR?

• group_column_supported_type now asks the factory: make_group_column(..).is_ok(). The 50-line allow-list and the lockstep comments are removed, so the two cannot disagree. Building a column only creates empty buffers, and this runs once per stream, not per batch.
• The pinning test is kept as the specification of which types take the column path and which fall back to rows; its doc comment is updated to say so. It gains the drifted Dictionary(Utf8, Int32) case, which fails on main (expected group_column_supported_type=false).

No behaviour change for any valid Arrow type.

What is the testing strategy for this PR?

Existing tests: group_column_supported_type_matches_make_group_column (supported and intentionally unsupported types, including nested and row-backed cases), the supported_schema_* tests and the rest of the multi_group_by tests.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.39%. Comparing base (a9579bd) to head (b46fc50).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25541      +/-   ##
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- Coverage   82.39%   82.39%   -0.01%     
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  Files        1138     1138              
  Lines      434623   434617       -6     
  Branches   434623   434617       -6     
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- Hits       358100   358090      -10     
  Misses      54856    54856              
- Partials    21667    21671       +4     

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[zhuqi-lucas]

LGTM, thanks @jayzhan211 !