tests: integration test for network-config schema on netplan systems

On Noble, python netplan bindings are available in python3-netplan deb
package. The presence of they python bindings and netplan.Parser import
allow cloud-int for parse and validate network-config version 2 schema.

Add an integration tests asserting both schema validation and
annotation of invalid network configuration on Ubuntu Noble.

On non-Noble environments, expect Skipping of network-config version: 2
message.

tests/integration_tests/cmd/test_schema.py

@@ -4,6 +4,7 @@
 import pytest
 
 from tests.integration_tests.instances import IntegrationInstance
+from tests.integration_tests.releases import CURRENT_RELEASE
 from tests.integration_tests.util import verify_clean_log
 
 USER_DATA = """\
@@ -13,6 +14,48 @@
 apt_reboot_if_required: false
 """
 
+NET_CFG_V1 = """\
+network:
+  version: 1
+  config:
+  - type: physical
+    name: eth0
+    subnets:
+      - type: dhcp
+"""
+NET_CFG_V1_INVALID = NET_CFG_V1.replace("config", "junk")
+NET_V1_ANNOTATED = """\
+network:		# E1,E2
+  version: 1
+  junk:
+  - type: physical
+    name: eth0
+    subnets:
+      - type: dhcp
+
+# Errors: -------------
+# E1: 'config' is a required property
+# E2: Additional properties are not allowed ('junk' was unexpected)"""
+
+NET_CFG_V2 = """\
+version: 2
+ethernets:
+  eth0:
+    dhcp4: true
+"""
+NET_CFG_V2_INVALID = NET_CFG_V2.replace("true", "bogus")
+NET_V2_ANNOTATED = """\
+---
+network:
+    ethernets:
+        eth0:
+            dhcp4: bogus		# E1
+    version: 2
+...
+
+# Errors: -------------
+# E1: Invalid netplan schema. Error in network definition: invalid boolean value 'bogus'"""  # noqa: E501
+
 
 @pytest.mark.user_data(USER_DATA)
 class TestSchemaDeprecations:
@@ -24,6 +67,65 @@ def test_clean_log(self, class_client: IntegrationInstance):
         assert "apt_update: Default: ``false``. Deprecated in version" in log
         assert "apt_upgrade: Default: ``false``. Deprecated in version" in log
 
+    def test_network_config_schema_validation(
+        self, class_client: IntegrationInstance
+    ):
+        content_responses = {
+            NET_CFG_V1: {"out": "Valid schema /root/net.yaml"},
+            NET_CFG_V1_INVALID: {
+                "out": "Invalid network-config /root/net.yaml",
+                "err": (
+                    "network: Additional properties are not allowed"
+                    " ('junk' was unexpected)"
+                ),
+                "annotate": NET_V1_ANNOTATED,
+            },
+        }
+        if CURRENT_RELEASE.series == "noble":
+            # Support for python3-netplan API available
+            content_responses[NET_CFG_V2] = {
+                "out": "Valid schema /root/net.yaml"
+            }
+            content_responses[NET_CFG_V2_INVALID] = {
+                "out": "Invalid network-config /root/net.yaml",
+                "err": (
+                    "Cloud config schema errors: format-l5.c20:"
+                    " Invalid netplan schema. Error in network definition:"
+                    " invalid boolean value 'bogus'"
+                ),
+                "annotate": NET_V2_ANNOTATED,
+            }
+        else:
+            # No python3-netplan API available skips validation
+            content_responses[NET_CFG_V2] = {
+                "out": (
+                    "Skipping network-config schema validation."
+                    " No network schema for version: 2"
+                )
+            }
+            content_responses[NET_CFG_V2_INVALID] = {
+                "out": (
+                    "Skipping network-config schema validation."
+                    " No network schema for version: 2"
+                )
+            }
+
+        for content, responses in content_responses.items():
+            class_client.write_to_file("/root/net.yaml", content)
+            result = class_client.execute(
+                "cloud-init schema --schema-type network-config"
+                " --config-file /root/net.yaml"
+            )
+            assert responses["out"] == result.stdout
+            if responses.get("err"):
+                assert responses["err"] in result.stderr
+            if responses.get("annotate"):
+                result = class_client.execute(
+                    "cloud-init schema --schema-type network-config"
+                    " --config-file /root/net.yaml --annotate"
+                )
+                assert responses["annotate"] in result.stdout
+
     def test_schema_deprecations(self, class_client: IntegrationInstance):
         """Test schema behavior with deprecated configs."""
         user_data_fn = "/root/user-data"