refactor: Optimize required_columns from BTreeSet to Vec in struct PushdownChecker (#19678)

## Which issue does this PR close?

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- Closes #19673.

## Rationale for this change

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## What changes are included in this PR?

- Changed
[row_filter.rs](https://github.com/apache/datafusion/blob/main/datafusion/datasource-parquet/src/row_filter.rs)
to use Vec instead of the BTreeSet

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## Are these changes tested?

Yes

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Author: kumarUjjawal

datafusion/datasource-parquet/src/row_filter.rs

@@ -276,7 +276,7 @@ struct PushdownChecker<'schema> {
     /// Does the expression reference any columns not present in the file schema?
     projected_columns: bool,
     /// Indices into the file schema of columns required to evaluate the expression.
-    required_columns: BTreeSet<usize>,
+    required_columns: Vec<usize>,
     /// Tracks the nested column behavior found during traversal.
     nested_behavior: NestedColumnSupport,
     /// Whether nested list columns are supported by the predicate semantics.
@@ -290,7 +290,7 @@ impl<'schema> PushdownChecker<'schema> {
         Self {
             non_primitive_columns: false,
             projected_columns: false,
-            required_columns: BTreeSet::default(),
+            required_columns: Vec::new(),
             nested_behavior: NestedColumnSupport::PrimitiveOnly,
             allow_list_columns,
             file_schema,
@@ -307,7 +307,8 @@ impl<'schema> PushdownChecker<'schema> {
             }
         };
 
-        self.required_columns.insert(idx);
+        // Duplicates are handled by dedup() in into_sorted_columns()
+        self.required_columns.push(idx);
         let data_type = self.file_schema.field(idx).data_type();
 
         if DataType::is_nested(data_type) {
@@ -355,6 +356,21 @@ impl<'schema> PushdownChecker<'schema> {
     fn prevents_pushdown(&self) -> bool {
         self.non_primitive_columns || self.projected_columns
     }
+
+    /// Consumes the checker and returns sorted, deduplicated column indices
+    /// wrapped in a `PushdownColumns` struct.
+    ///
+    /// This method sorts the column indices and removes duplicates. The sort
+    /// is required because downstream code relies on column indices being in
+    /// ascending order for correct schema projection.
+    fn into_sorted_columns(mut self) -> PushdownColumns {
+        self.required_columns.sort_unstable();
+        self.required_columns.dedup();
+        PushdownColumns {
+            required_columns: self.required_columns,
+            nested: self.nested_behavior,
+        }
+    }
 }
 
 impl TreeNodeVisitor<'_> for PushdownChecker<'_> {
@@ -390,9 +406,13 @@ enum NestedColumnSupport {
     Unsupported,
 }
 
+/// Result of checking which columns are required for filter pushdown.
 #[derive(Debug)]
 struct PushdownColumns {
-    required_columns: BTreeSet<usize>,
+    /// Sorted, unique column indices into the file schema required to evaluate
+    /// the filter expression. Must be in ascending order for correct schema
+    /// projection matching.
+    required_columns: Vec<usize>,
     nested: NestedColumnSupport,
 }
 
@@ -411,10 +431,7 @@ fn pushdown_columns(
     let allow_list_columns = supports_list_predicates(expr);
     let mut checker = PushdownChecker::new(file_schema, allow_list_columns);
     expr.visit(&mut checker)?;
-    Ok((!checker.prevents_pushdown()).then_some(PushdownColumns {
-        required_columns: checker.required_columns,
-        nested: checker.nested_behavior,
-    }))
+    Ok((!checker.prevents_pushdown()).then(|| checker.into_sorted_columns()))
 }
 
 fn leaf_indices_for_roots(