Fix computing the maximum merged PR (#7127)

Assisted-by: Antigravity with Gemini

github_tools/check_dependent_pr.py

@@ -118,7 +118,11 @@ _QUERY_LABEL = """
 _QUERY_MAX_MERGED_PR = """
 {
   repository(owner: "carbon-language", name: "carbon-lang") {
-    pullRequests(states: MERGED, first: 1) {
+    pullRequests(
+      states: MERGED
+      orderBy: {field: CREATED_AT, direction: DESC}
+      first: 1
+    ) {
       nodes {
         number
       }

github_tools/check_dependent_pr_test.py

@@ -499,6 +499,16 @@ class TestCheckDependentPR(unittest.TestCase):
             _OID2, "pending", "This PR has open dependencies: #1"
         )
 
+    def test_query_max_merged_pr_explicit_orderBy_and_first_one(self) -> None:
+        self.assertIn(
+            "orderBy: {field: CREATED_AT, direction: DESC}",
+            check_dependent_pr._QUERY_MAX_MERGED_PR,
+        )
+        self.assertIn(
+            "first: 1",
+            check_dependent_pr._QUERY_MAX_MERGED_PR,
+        )
+
 
 if __name__ == "__main__":
     unittest.main()