fix: Add missing ColumnRef import in window_functions/mod.rs

The rust-build-fixer agent removed the ColumnRef import while fixing
clippy warnings, but this import is actually needed for the test at
line 770.

Changes:
- src/sql/window_functions/mod.rs: Added ColumnRef to imports
- src/query_plan/into_clause_remover.rs: Removed unused SelectItem import

Fixes CI build error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: TimelordUK

src/query_plan/into_clause_remover.rs

@@ -196,7 +196,7 @@ impl IntoClauseRemover {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::sql::parser::ast::{IntoTable, SelectItem};
+    use crate::sql::parser::ast::IntoTable;
 
     #[test]
     fn test_remove_simple_into() {

src/sql/window_functions/mod.rs

@@ -6,7 +6,7 @@ use std::collections::HashMap;
 use std::sync::Arc;
 
 use crate::data::datatable::DataValue;
-use crate::sql::parser::ast::{SqlExpression, WindowSpec};
+use crate::sql::parser::ast::{ColumnRef, SqlExpression, WindowSpec};
 use crate::sql::window_context::WindowContext;
 
 /// Window function computation trait