[#21878] extensive test for multi-dictionary column group bys

[Rich-T-kid]

Which issue does this PR close?

works towards closing #21878. But also adds test coverage for existing code that wasnt there before. There were no test for dictionaries specifically for groupValueRows

Rationale for this change

There was a lack of test that focused on dictionary arrays in GroupValueRows. I also would like to keep the test separate from the implementation (for #21878) so that its easier to review.

What changes are included in this PR?

• Adds a dedicated test module (multi_group_by/dictionary.rs) covering correctness of multi-column dictionary GROUP BY using the GroupValuesRows path
• Tests are organized into three modules: general_test (uniqueness, null semantics, multi-type composition), dictionary_encoding (key-index shuffling, bloated/non-normalized dictionaries, mixed key types), and streaming (EmitTo::First(n) renumbering and EmitTo::All reingest)
• Adds two SQL logic tests to dictionary.slt covering multi-column dictionary GROUP BY with mixed key/value types (Dict<Int8,Utf8>, Dict<Int32,LargeUtf8>, Dict<Int8,Utf8View>). these are the first SLT tests where the grouping keys themselves are dictionary columns

Are these changes tested?

The changes are test 😆

Are there any user-facing changes?

No

[Rich-T-kid]

I tried to make each test function as declarative as possible.