[fix](agg) Fix distinct aggregate argument grouping and strategy selection (#65368)

### What problem does this PR solve?

Problem Summary:

The Nereids distinct aggregate planning logic had several correctness
issues:

1. `CheckMultiDistinct` determined distinct groups from all aggregate
arguments instead of the semantic DISTINCT arguments. This incorrectly
treated separators and ORDER BY expressions as deduplication keys and
mishandled aggregates whose DISTINCT argument sets were equivalent but
ordered differently, such as `COUNT(DISTINCT a, b)` and `COUNT(DISTINCT b, a)`.

2. `GROUP_CONCAT` accepted a column expression as its separator. During
distinct aggregate rewriting, this extra input could incorrectly participate
in deduplication even though DISTINCT semantics only apply to the value argument.

3. Forced `agg_phase=3/4` could override `GROUP_CONCAT(DISTINCT ...
ORDER BY ...)`'s requirement to use the multi-distinct implementation.
Conversely, early logical splitting could prevent forced three/four-phase
aggregation from being honored.

4. `AggregateUtils#getDistinctNamedExpr` collected all aggregate
arguments rather than only semantic DISTINCT arguments.

This PR:

- Compares DISTINCT aggregates by their semantic DISTINCT argument groups.
- Treats equivalent argument sets as one distinct group.
- Requires the separator of `GROUP_CONCAT` and
`MULTI_DISTINCT_GROUP_CONCAT` to be a constant.
- Introduces explicit strategy selection between logical splitting,
multi-distinct aggregation, and Cascades multi-phase splitting.
- Preserves `mustUseMultiDistinctAgg()` when `agg_phase` is forced.
- Lets `agg_phase=3/4` retain the original DISTINCT aggregate for Cascades phase planning.
- Rejects queries that combine `GROUP_CONCAT(DISTINCT ... ORDER BY ...)` with a multi-argument `COUNT(DISTINCT ...)`, because their required aggregation strategies are incompatible.
- Uses `AggregateFunction#getDistinctArguments()` when collecting distinct deduplication keys.

### Release note

`GROUP_CONCAT` and `MULTI_DISTINCT_GROUP_CONCAT` now require the
separator argument to be a constant.

Queries combining `GROUP_CONCAT(DISTINCT ... ORDER BY ...)` with a
multi-argument `COUNT(DISTINCT ...)` are rejected with a clear analysis
error.