Allow filter pushdown through AggregateExec (#18404)

## Which issue does this PR close?

- Closes #18399

## Rationale for this change

Right now filters cannot pass through `AggregateExec` nodes, preventing
filter pushdown optimization in queries with GROUP BY/DISTINCT
operations.

## What changes are included in this PR?

- Implemented `gather_filters_for_pushdown()` for `AggregateExec` that
allows filters on grouping columns to pass through to children
- Supports both Pre phase (static filters) and Post phase (dynamic
filters from joins)

Essentially, filter will pass through in the scenarios @asolimando
mentioned
[here](#18399 (comment))


## Are these changes tested?

Yes, added three tests:
- `test_aggregate_filter_pushdown`: Positive case with aggregate
functions
- `test_no_pushdown_aggregate_filter_on_non_grouping_column`: Negative
case ensuring filters on aggregate results are not pushed

## Are there any user-facing changes?

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Author: LiaCastaneda