tests/rptest: cover unmatched-route 404 shape on REST proxy and SR

Asserts that GET on a bogus path returns HTTP 404 with the
{"error_code": 404, "message": "..."} envelope clients expect, rather
than Seastar's fallback {"message": "Not found", "code": 404}. Two
assertions, one per port:

- Schema registry (8081) in schema_registry_test.py
- REST proxy (8082) in pandaproxy_test.py

Both subsystems use pandaproxy::server, so the fix in
pandaproxy::server::start() lands on both. Locking in both wire
formats end-to-end is the test that proves the default_404_handler
registration works for both consumers. Removing the
add_default_handler line would regress the body shape and fail both
assertions in CI.

Author: nguyen-andrew

tests/rptest/tests/pandaproxy_test.py

@@ -502,6 +502,25 @@ def test_get_brokers(self):
 
         assert sorted(brokers) == sorted(node_idxs)
 
+    @cluster(num_nodes=3)
+    def test_unmatched_route_404_shape(self):
+        """
+        Unmatched routes on REST proxy must return the standard
+        {"error_code": <int>, "message": "..."} JSON envelope, not Seastar's
+        fallback {"message": "Not found", "code": 404}. The fix lives in
+        pandaproxy::server, which both REST proxy and schema registry share,
+        so this assertion mirrors the schema-registry-side test.
+        """
+        result_raw = requests.get(f"{self._base_uri()}/_no_such_path")
+        assert result_raw.status_code == requests.codes.not_found, (
+            f"expected 404, got {result_raw.status_code}: {result_raw.text}"
+        )
+
+        body = result_raw.json()
+        assert "error_code" in body, f"expected error_code field, got body={body}"
+        assert body["error_code"] == 404, f"expected error_code=404, got body={body}"
+        assert "message" in body, f"expected message field, got body={body}"
+
     @cluster(num_nodes=3)
     def test_list_topics_validation(self):
         """

tests/rptest/tests/schema_registry_test.py

@@ -1563,6 +1563,25 @@ def test_schemas_types(self):
         result = result_raw.json()
         assert set(result) == {"JSON", "PROTOBUF", "AVRO"}
 
+    @cluster(num_nodes=3)
+    def test_unmatched_route_404_shape(self):
+        """
+        Unmatched routes on schema registry must return the standard
+        {"error_code": <int>, "message": "..."} JSON envelope, not Seastar's
+        fallback {"message": "Not found", "code": 404}. SR clients parse
+        `error_code`; without this shape, fallback paths that inspect 404
+        bodies produce a 0-coded error.
+        """
+        result_raw = self.sr_client.request("GET", "_no_such_path")
+        assert result_raw.status_code == requests.codes.not_found, (
+            f"expected 404, got {result_raw.status_code}: {result_raw.text}"
+        )
+
+        body = result_raw.json()
+        assert "error_code" in body, f"expected error_code field, got body={body}"
+        assert body["error_code"] == 404, f"expected error_code=404, got body={body}"
+        assert "message" in body, f"expected message field, got body={body}"
+
     @cluster(num_nodes=3)
     def test_get_schema_id_versions(self):
         """