test: exercise concat_many with all-columnar inputs

The concat_many teardown made the Vec arm unreachable, so a mixed
Vec/Columnar input can no longer reach concat_many. Feed two columnar
edges instead, which is the only shape producers now emit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: antiguru

src/compute/src/render/columnar.rs

@@ -523,7 +523,7 @@ mod tests {
     }
 
     #[mz_ore::test]
-    fn concat_many_mixed_upgrades_to_columnar() {
+    fn concat_many_concatenates_columnar() {
         let rows = test_rows();
         let expected: Vec<_> = {
             let mut updates: Vec<_> = rows
@@ -542,7 +542,7 @@ mod tests {
                 let edge = CollectionEdge::concat_many(
                     scope,
                     [
-                        CollectionEdge::Vec(collection1),
+                        CollectionEdge::Columnar(vec_to_columnar(collection1)),
                         CollectionEdge::Columnar(vec_to_columnar(collection2)),
                     ],
                 );