chore: add functional test for streaming table liquid clustering ALTER path (#1347)

## Summary

Regression test for #1329 (fix merged in #1330)

## Test plan

- [x] CI passes (unit tests)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: sd-db

tests/functional/adapter/streaming_tables/test_st_basic.py

@@ -4,8 +4,10 @@
 from dbt.adapters.base.relation import BaseRelation
 from dbt.tests import util
 from dbt.tests.adapter.materialized_view.files import MY_SEED, MY_TABLE, MY_VIEW
+from dbt_common.contracts.config.materialization import OnConfigurationChangeOption
 
 from dbt.adapters.databricks.relation import DatabricksRelationType
+from dbt.adapters.databricks.relation_configs.streaming_table import StreamingTableConfig
 from tests.functional.adapter.streaming_tables import fixtures
 
 
@@ -300,6 +302,55 @@ def test_create_with_liquid_clustering_config(self, project):
         assert relation_type == "streaming_table"
 
 
+@pytest.mark.dlt
+@pytest.mark.skip_profile("databricks_cluster", "databricks_uc_cluster")
+class TestStreamingTableLiquidClusteringChanges:
+    """Test liquid clustering changes on existing streaming tables (#1329)."""
+
+    @pytest.fixture(scope="class")
+    def seeds(self):
+        return {"my_seed.csv": MY_SEED}
+
+    @pytest.fixture(scope="class")
+    def models(self):
+        yield {
+            "liquid_clustered_st.sql": fixtures.liquid_clustered_st,
+            "schema.yml": fixtures.liquid_clustered_st_schema_v1,
+        }
+
+    @pytest.fixture(scope="class")
+    def project_config_update(self):
+        return {"models": {"on_configuration_change": OnConfigurationChangeOption.Apply.value}}
+
+    @pytest.fixture(scope="class")
+    def liquid_clustered_st(self, project) -> BaseRelation:
+        return project.adapter.Relation.create(
+            identifier="liquid_clustered_st",
+            schema=project.test_schema,
+            database=project.database,
+            type=DatabricksRelationType.StreamingTable,
+        )
+
+    @pytest.fixture(scope="class", autouse=True)
+    def setup(self, project, liquid_clustered_st):
+        util.run_dbt(["seed"])
+        util.run_dbt(["run", "--models", liquid_clustered_st.identifier, "--full-refresh"])
+
+        yield
+
+        project.run_sql(f"drop schema if exists {project.test_schema} cascade")
+
+    def test_liquid_clustering_change_is_applied(self, project, liquid_clustered_st):
+        """Changing liquid_clustered_by on an existing ST should apply via ALTER."""
+        util.write_file(fixtures.liquid_clustered_st_schema_v2, "models", "schema.yml")
+        util.run_dbt(["run", "--models", liquid_clustered_st.identifier])
+
+        with util.get_connection(project.adapter):
+            config = project.adapter.get_relation_config(liquid_clustered_st)
+        assert isinstance(config, StreamingTableConfig)
+        assert config.config["liquid_clustering"].cluster_by == ["id", "value"]
+
+
 @pytest.mark.dlt
 @pytest.mark.skip_profile("databricks_cluster", "databricks_uc_cluster")
 class TestStreamingTablesFromFiles(TestStreamingTablesMixin):