test: add negative tests that are not covered (#1265)

* test: add negative tests that are not covered

Signed-off-by: Debarati Basu-Nag <dbasunag@redhat.com>

* fix: address coderabbit's review comments

Signed-off-by: Debarati Basu-Nag <dbasunag@redhat.com>

---------

Signed-off-by: Debarati Basu-Nag <dbasunag@redhat.com>

Author: dbasunag

tests/model_registry/model_registry/python_client/signing/test_signing_negative.py

@@ -0,0 +1,44 @@
+"""Negative tests for model signing failure scenarios (TC-011b, TC-011c)."""
+
+from pathlib import Path
+
+import pytest
+from model_registry.signing.exceptions import SigningError
+from simple_logger.logger import get_logger
+
+LOGGER = get_logger(name=__name__)
+
+pytestmark = pytest.mark.usefixtures("skip_if_not_managed_cluster", "tas_connection_type")
+
+INVALID_IMAGE_REF = "quay.io/invalid-no-access-repo/nonexistent-image:latest"
+
+
+@pytest.mark.tier3
+@pytest.mark.usefixtures("set_environment_variables")
+class TestSigningNegative:
+    """Negative test suite for signing failure scenarios."""
+
+    def test_sign_image_invalid_registry_credentials(self, signer):
+        """TC-011b: Sign container image with invalid registry credentials.
+
+        Verifies that signing an image to a registry where the user has no write
+        permissions fails with a SigningError indicating an authentication/authorization issue.
+        """
+        LOGGER.info(f"Attempting to sign image with no write access: {INVALID_IMAGE_REF}")
+        with pytest.raises(SigningError):
+            signer.sign_image(image=INVALID_IMAGE_REF)
+        LOGGER.info("SigningError raised as expected for invalid registry credentials")
+
+    def test_verify_model_missing_signature(self, signer, tmp_path: Path):
+        """TC-011c: Verify model with missing signature file.
+
+        Verifies that attempting to verify a model directory that has no model.sig
+        file fails with a FileNotFoundError indicating the signature is missing.
+        """
+        model_file = tmp_path / "model.onnx"
+        model_file.write_bytes(data=b"fake onnx model content for testing")
+
+        LOGGER.info(f"Attempting to verify model without signature: {tmp_path}")
+        with pytest.raises(FileNotFoundError, match=r"model\.sig"):
+            signer.verify_model(model_path=str(tmp_path))
+        LOGGER.info("FileNotFoundError raised as expected for missing signature")