fix: Address CodeRabbit nitpicks

- Add value assertion in test_parse_update_single_column for completeness
- Add duplicate column assignment validation in UPDATE executor
- Prevent same column from being assigned multiple times in single UPDATE

Author: sshekhar563

nexum_core/src/executor/mod.rs

@@ -249,6 +249,18 @@ impl Executor {
                 let column_names: Vec<String> =
                     schema.columns.iter().map(|c| c.name.clone()).collect();
 
+                // Check for duplicate column assignments
+                let mut seen_columns: std::collections::HashSet<&str> =
+                    std::collections::HashSet::new();
+                for (col_name, _) in &assignments {
+                    if !seen_columns.insert(col_name.as_str()) {
+                        return Err(StorageError::ReadError(format!(
+                            "Duplicate column assignment for '{}'",
+                            col_name
+                        )));
+                    }
+                }
+
                 // Build column index map for assignments
                 let mut assignment_indices: Vec<(usize, Value)> = Vec::new();
                 for (col_name, new_value) in &assignments {

nexum_core/src/sql/parser.rs

@@ -278,6 +278,7 @@ mod tests {
                 assert_eq!(table, "users");
                 assert_eq!(assignments.len(), 1);
                 assert_eq!(assignments[0].0, "name");
+                assert_eq!(assignments[0].1, Value::Text("Bob".to_string()));
                 assert!(where_clause.is_some());
             }
             _ => panic!("Expected Update statement"),