Re-enable COUNT for mixed Spark partial / Comet final aggregate execution

## What is the problem the feature request solves?

When #2894 was first addressed by #2994, `COUNT` was included in the set of aggregates safe to run with mixed Spark partial / Comet final execution alongside `MIN`, `MAX`, and the bitwise aggregates. The follow-up branch for that work removed `COUNT` from the safe set after two regressions surfaced. As a result, the TPC-DS coverage gains in #2994 (which were almost entirely driven by `COUNT`) are not realized in the carve-out PR.

This issue tracks investigating and re-enabling `COUNT` for mixed Spark partial / Comet final execution.

## Known blockers

The two specific regressions that caused `COUNT` to be excluded:

1. **AQE `PropagateEmptyRelationAfterAQE`**. The rule matches `BaseAggregateExec` only, not `CometHashAggregateExec`. When the partial runs in Spark and the final runs in Comet, the rule no longer fires for the final stage, which changes results in some queries.

2. **Spark 4.0 count-bug decorrelation**. The decorrelation rewrite for correlated `IN` subqueries drops a row in the OR pattern in `in-count-bug.sql` when the partial/final aggregate stages are split between Spark and Comet.

## Suggested approach

- Reproduce both regressions with `COUNT` re-added to `supportsMixedPartialFinal` in `aggregates.scala`.
- For (1), evaluate whether the right fix is upstream (extend `PropagateEmptyRelationAfterAQE` to recognize Comet aggregate exec) or in Comet (e.g., guard mixed-COUNT when AQE is enabled, or insert a wrapper Spark aggregate node).
- For (2), determine whether the row-drop is specific to Spark 4.0 decorrelation interacting with mixed aggregate stages, and whether a targeted guard is preferable to broad fallback.
- Once both are addressed, re-add `override def supportsMixedPartialFinal: Boolean = true` to `CometCount` and regenerate TPC-DS golden files.

## Related

- Parent: #2892
- Original: #2894 (closed by #2994 follow-up)
- Adjacent: #1389 (AQE materializing unsupported final HashAggregate)