feat: add GroupColumn support for Interval in multi-column GROUP BY

[tohuya6]

Which issue does this PR close?

• Part of EPIC: complete GroupValuesColumn type coverage (nested types + remaining primitives) #22715.

Rationale for this change

multi_group_by::group_column_supported_type gates which GROUP BY columns may
use the column-wise GroupValuesColumn fast path, and the gate is
all-or-nothing: a single unsupported column forces the entire grouping onto
the byte-encoded GroupValuesRows fallback, even when every other key column
would have qualified. An Interval key triggers exactly that today.

All three Interval units reuse the existing PrimitiveGroupValueBuilder with
no new builder type and no new HashValue impl — the i32 / IntervalDayTime
/ IntervalMonthDayNano natives already implement it.

What changes are included in this PR?

• Dispatch the three Interval*Type units in make_group_column. IntervalUnit
  has exactly three variants, so the match is exhaustive with no fallback arm
  (unlike Time32/Time64, which must reject invalid unit combinations).
• Accept Interval(_) in group_column_supported_type.
• Extend the group_column_supported_type ↔ make_group_column consistency
  fuzz with the three Interval units.
• Add an (Interval, Int32) group-count benchmark to benches/multi_group_by.rs.

Are these changes tested?

Yes.

• New unit test: an Interval key stays on the GroupValuesColumn path, dedups
  including nulls, keeps "1 month" and "30 days" as distinct groups (no
  cross-unit folding), and round-trips with the Interval output type preserved.
• The consistency fuzz now asserts all three Interval units route through the
  dispatcher.
• New single- and multi-column Interval GROUP BY coverage in aggregate.slt.

Are there any user-facing changes?

No API changes. GROUP BY queries with an Interval key now use the
column-wise fast path instead of the row-encoded fallback; results are unchanged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.71%. Comparing base (5de7f1d) to head (b9753c4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/aggregates/group_values/multi_group_by/mod.rs	94.73%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23786      +/-   ##
==========================================
- Coverage   80.71%   80.71%   -0.01%     
==========================================
  Files        1089     1089              
  Lines      368760   368798      +38     
  Branches   368760   368798      +38     
==========================================
+ Hits       297647   297666      +19     
- Misses      53372    53382      +10     
- Partials    17741    17750       +9     

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