api: stub out SignImage endpoint

BUG=b:296089271
TEST=./run_tests api/controller/image_unittest.py::SignImageTest
TEST=./api/contrib/call_scripts/image__sign_image

Change-Id: Ic71b9b66572431b975a75e2980c49bdb1a48e925
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4828447
Reviewed-by: Benjamin Shai <bshai@google.com>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
diff --git a/api/controller/image_unittest.py b/api/controller/image_unittest.py
index 92a5cf3..c93d08c 100644
--- a/api/controller/image_unittest.py
+++ b/api/controller/image_unittest.py
@@ -790,3 +790,21 @@
             request, self._GetResponse, self.api_config
         )
         self.assertEqual(rc, controller.RETURN_CODE_COMPLETED_UNSUCCESSFULLY)
+
+
+class SignImageTest(cros_test_lib.MockTestCase, api_config.ApiConfigMixin):
+    """Sign image test."""
+
+    def testValidateOnly(self):
+        """Check that a validate only call does not execute any logic."""
+        req = image_pb2.SignImageRequest()
+        resp = image_pb2.SignImageResponse()
+        rc = image_controller.SignImage(req, resp, self.validate_only_config)
+        self.assertEqual(rc, controller.RETURN_CODE_VALID_INPUT)
+
+    def testSuccess(self):
+        """Check that the endpoint finishes successfully."""
+        req = image_pb2.SignImageRequest()
+        resp = image_pb2.SignImageResponse()
+        rc = image_controller.SignImage(req, resp, self.mock_call_config)
+        self.assertEqual(rc, controller.RETURN_CODE_SUCCESS)