CoalescePartitionsExec::cardinality_effect returns Equal when fetch is set #26050

Describe the bug

CoalescePartitionsExec::cardinality_effect always returns CardinalityEffect::Equal, which is documented as "the operator is guaranteed to produce exactly one row for each input row". When fetch is set, the operator can produce fewer rows than its input, so it should return CardinalityEffect::LowerEqual, as SortExec and SortPreservingMergeExec already do when they have a fetch.

Any code that relies on cardinality_effect gets the wrong answer for this operator. For example, PassthroughStatisticsProvider returns the input statistics unchanged for operators with CardinalityEffect::Equal, so a CoalescePartitionsExec with fetch = 10 over 1000 rows reports Exact(1000) instead of Exact(10).

To Reproduce

let plan = CoalescePartitionsExec::new(input).with_fetch(Some(10));
assert!(matches!(plan.cardinality_effect(), CardinalityEffect::LowerEqual)); // fails: Equal

Expected behavior

cardinality_effect returns CardinalityEffect::LowerEqual when fetch is set, and CardinalityEffect::Equal otherwise.

Additional context

Other readers of cardinality_effect are the physical optimizer rules sort_pushdown, topk_aggregation and limit_pushdown_past_window. I did not find a query that returns wrong results because of this, and no sqllogictest plan changes with the fix.