Prefer last comment when there are multiple to choose from

pr-checks/post-comment.test.ts

@@ -193,7 +193,7 @@ describe("findExistingComment", async () => {
       1,
       "marker",
     );
-    assert.equal(id, 2);
+    assert.equal(id, 3);
   });
 
   await it("returns undefined when no comment matches", async () => {

pr-checks/post-comment.ts

@@ -112,7 +112,7 @@ export async function findExistingComment(
         "Use a unique marker for each distinct managed comment to avoid this situation.",
     );
   }
-  return markedComments[0]?.id;
+  return markedComments.at(-1)?.id;
 }
 
 /**