fix(mssql): skip dbo on drop schema (#6026)

Signed-off-by: Cortland Goffena <30168413+cmgoffena13@users.noreply.github.com>

Author: cmgoffena13

sqlmesh/core/engine_adapter/mssql.py

@@ -180,7 +180,10 @@ def drop_schema(
         **drop_args: t.Dict[str, exp.Expr],
     ) -> None:
         """
-        MsSql doesn't support CASCADE clause and drops schemas unconditionally.
+        MsSql doesn't support CASCADE clause so objects are dropped individually.
+
+        SQL Server also forbids dropping the built-in ``dbo`` schema (error 15150).
+        Objects inside it are still dropped when cascade=True, but the schema itself is left in place.
         """
         if cascade:
             objects = self._get_data_objects(schema_name)
@@ -199,6 +202,15 @@ def drop_schema(
                         object_table,
                         exists=ignore_if_not_exists,
                     )
+
+        schema = (
+            (schema_name.db or schema_name.name)
+            if isinstance(schema_name, exp.Table)
+            else schema_name
+        )
+        if schema.lower() == "dbo":
+            return
+
         super().drop_schema(schema_name, ignore_if_not_exists=ignore_if_not_exists, cascade=False)
 
     def merge(

tests/core/engine_adapter/test_mssql.py

@@ -714,6 +714,38 @@ def test_drop_schema(make_mocked_engine_adapter: t.Callable):
     ]
 
 
+def test_drop_schema_skips_dbo(make_mocked_engine_adapter: t.Callable):
+    adapter = make_mocked_engine_adapter(MSSQLEngineAdapter)
+
+    adapter._get_data_objects = mock.Mock()
+    adapter._get_data_objects.return_value = [
+        DataObject(
+            catalog="test_catalog",
+            schema="dbo",
+            name="test_view",
+            type=DataObjectType.from_str("VIEW"),
+        )
+    ]
+
+    # String "dbo" path (original coverage)
+    adapter.drop_schema("dbo", cascade=True)
+
+    sql_calls = to_sql_calls(adapter)
+    assert """DROP VIEW IF EXISTS [dbo].[test_view];""" in sql_calls
+    assert """DROP SCHEMA IF EXISTS [dbo];""" not in sql_calls
+
+    # exp.to_table("dbo") path — schema ends up in .name, not .db
+    adapter2 = make_mocked_engine_adapter(MSSQLEngineAdapter)
+    adapter2._get_data_objects = mock.Mock()
+    adapter2._get_data_objects.return_value = []
+
+    schema_name = exp.to_table("dbo", dialect="tsql")
+    adapter2.drop_schema(schema_name, cascade=False)
+
+    sql_calls2 = to_sql_calls(adapter2)
+    assert """DROP SCHEMA IF EXISTS [dbo];""" not in sql_calls2
+
+
 def test_drop_schema_with_special_identifiers(make_mocked_engine_adapter: t.Callable):
     adapter = make_mocked_engine_adapter(MSSQLEngineAdapter)