refactor: use VLP_CTE_FROM_ALIAS constant instead of hardcoded 't'

Addresses Copilot review feedback: replace magic string with named constant
for better maintainability.

Author: genezhang

src/render_plan/plan_builder_utils.rs

@@ -4419,9 +4419,9 @@ pub(crate) fn rewrite_vlp_union_branch_aliases(
     let path_function_mappings: HashMap<String, String> = filtered_mappings
         .iter()
         .filter(|(from_alias, _to_alias)| {
-            // Only keep "t" mapping (for path functions like length(path))
+            // Only keep VLP_CTE_FROM_ALIAS mapping (for path functions like length(path))
             // Exclude endpoint node aliases
-            *from_alias == "t"
+            *from_alias == VLP_CTE_FROM_ALIAS
         })
         .map(|(k, v)| (k.clone(), v.clone()))
         .collect();

src/server/query_context.rs

@@ -329,10 +329,7 @@ mod tests {
     async fn test_denormalized_aliases() {
         with_query_context(QueryContext::empty(), async {
             register_denormalized_alias("d", "r2");
-            assert_eq!(
-                get_denormalized_alias_mapping("d"),
-                Some("r2".to_string())
-            );
+            assert_eq!(get_denormalized_alias_mapping("d"), Some("r2".to_string()));
             assert_eq!(get_denormalized_alias_mapping("unknown"), None);
         })
         .await;