[Bug] MultiDistinct can OOM on group-by with high-NDV or unknown-statistics distinct arguments

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

master / 4.0

### What's Wrong?

The `MultiDistinct` strategy keeps a per-group hash set of every distinct value on a single node. For a group-by query whose distinct argument is near-unique (or whose statistics are unknown and could be near-unique), one node can end up holding a whole group's value set and OOM, e.g.:

```sql
select count(distinct payment_id), ... from t group by merchant_id
```

The no-group-by branch of `DistinctAggStrategySelector.shouldUseMultiDistinct` already guards against this by checking distinct-argument NDV and routing unknown-statistics cases to the CTE split strategy. The group-by branch does not: it neither checks distinct-argument NDV, nor treats unknown distinct-argument statistics as risky, and on unknown group-by statistics it unconditionally chooses `MultiDistinct`.

### What You Expected?

For a group-by query whose distinct argument is near-unique relative to input rows, or whose distinct argument (or a slot it reads) has unknown statistics, the planner should route to the CTE split strategy (which redistributes on the distinct key) to avoid the single-node OOM — mirroring the no-group-by branch.

### How to Reproduce?

Run a group-by aggregation with one or more `count(distinct ...)` over a near-unique column (or a column wrapped in `if(cond, col, null)` where `col` is unanalyzed), and observe that `MultiDistinct` is chosen and can OOM under a low-cardinality group-by key.

### Anything Else?

Fix (group-by branch of `shouldUseMultiDistinct`), matching the no-group-by branch:
1. `hasHighNdvDistinctArgument`: force CTE split when any distinct argument's estimated NDV >= `row * MID_CARDINALITY_THRESHOLD`.
2. `hasUnknownNdvDistinctArgument`: force CTE split when a distinct argument's estimate is unknown, or any input slot it reads has absent/unknown statistics (the per-slot scan is required because `ExpressionEstimation.visitIf` fabricates a non-unknown NDV for `if(cond, col, null)` even when `col` is unanalyzed).
3. When group-by statistics are unknown (reached only when every distinct argument is confirmed low-NDV), prefer CTE split for a single group-by key and keep `MultiDistinct` for >= 2 keys.

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start at DistinctAggStrategySelector.shouldUseMultiDistinct and compare the group-by branch with the no-group-by branch. Trace how distinct-argument and group-by statistics are estimated, then verify that high or unknown distinct NDV selects CTE split, while unknown group-by statistics select CTE split for one key and preserve MultiDistinct for two or more keys.

Written by the indexing model from the issue text.