fix: FilterExec should drop projection when apply projection pushdown

[haohuaijin]

Which issue does this PR close?

• Closes regression: ProjectionPushdown failed with project index 2 out of bounds, max field 2 #21459

Rationale for this change

When a ProjectionExec sits on top of a FilterExec that already carries an explicit projection, the ProjectionPushdown optimizer attempts to swap them via try_swapping_with_projection. The swap replaces the FilterExec's input with the narrower ProjectionExec, but FilterExecBuilder::from(self) carried over the old projection indices (e.g. [0, 1, 2]). After the swap the new input only has the columns selected by the ProjectionExec (e.g. 2 columns), so .build() tries to validate the stale projection against the narrower schema and panics with "project index 2 out of bounds, max field 2".

What changes are included in this PR?

In FilterExec::try_swapping_with_projection, after replacing the input with the narrower ProjectionExec, clear the FilterExec's own projection via .apply_projection(None). The ProjectionExec that is now the input already handles column selection, so the FilterExec no longer needs its own projection.

Are these changes tested?

yes, add test case

Are there any user-facing changes?

[alamb]

Thanks @haohuaijin

the fix makes sense to me, but I think it would be better to try and add some higher level tests if possible

[alamb]

Thanks @haohuaijin

[haohuaijin]

Thanks for your quick reviews @alamb