Improve memory accounting for ArrowBytesViewMap (#20077)

## Which issue does this PR close?

- Closes #20074

## Rationale for this change

ArrowBytesViewMap was previously accounting for the logical number of
null bits when reporting memory usage. This under-reported memory
consumption is because NullBufferBuilder may allocate more memory than
is currently used.

Memory accounting in DataFusion is expected to reflect allocated memory
rather than logical usage to ensure accurate memory tracking.

## What changes are included in this PR?

- Update ArrowBytesViewMap::size to use
NullBufferBuilder::allocated_size instead of calculating size from the
number of used null bits.

## Are these changes tested?

- Yes. Existing tests were run:
  - cargo test -p datafusion-physical-expr-common

## Are there any user-facing changes?

- No. This change only affects internal memory accounting and does not
alter query behavior or public APIs.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: vigneshsiva11

datafusion/physical-expr-common/src/binary_view_map.rs

@@ -437,7 +437,7 @@ where
         let views_size = self.views.len() * size_of::<u128>();
         let in_progress_size = self.in_progress.capacity();
         let completed_size: usize = self.completed.iter().map(|b| b.len()).sum();
-        let nulls_size = self.nulls.len() / 8;
+        let nulls_size = self.nulls.allocated_size();
 
         self.map_size
             + views_size