[Fix] Don't fail delete when databricks_system_schema can be disabled only by Databricks (#4727)

## Changes
<!-- Summary of your changes that are easy to understand -->

Also rewrote corresponding tests to use the mock framework from Go SDK

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [x] `make test` run locally
- [x] relevant change in `docs/` folder
- [x] covered with integration tests in `internal/acceptance`
- [x] using Go SDK
- [ ] using TF Plugin Framework

Author: alexott

NEXT_CHANGELOG.md

@@ -6,6 +6,8 @@
 
 ### Bug Fixes
 
+ * Don't fail delete when `databricks_system_schema` can be disabled only by Databricks [#4727](https://github.com/databricks/terraform-provider-databricks/pull/4727)
+
 ### Documentation
 
 ### Exporter

catalog/resource_system_schema.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"strings"
 
+	"github.com/databricks/databricks-sdk-go"
 	"github.com/databricks/databricks-sdk-go/service/catalog"
 	"github.com/databricks/terraform-provider-databricks/common"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
@@ -70,10 +71,7 @@ func ResourceSystemSchema() common.Resource {
 				log.Printf("[WARN] %s is auto enabled, ignoring it", old)
 				return nil
 			}
-			err = w.SystemSchemas.Disable(ctx, catalog.DisableRequest{
-				MetastoreId: metastoreSummary.MetastoreId,
-				SchemaName:  old,
-			})
+			err = safeDisable(ctx, w, metastoreSummary.MetastoreId, old)
 			if err != nil {
 				return err
 			}
@@ -143,10 +141,22 @@ func ResourceSystemSchema() common.Resource {
 			if err != nil {
 				return err
 			}
-			return w.SystemSchemas.Disable(ctx, catalog.DisableRequest{
-				MetastoreId: metastoreSummary.MetastoreId,
-				SchemaName:  schemaName,
-			})
+			return safeDisable(ctx, w, metastoreSummary.MetastoreId, schemaName)
 		},
 	}
 }
+
+func safeDisable(ctx context.Context, w *databricks.WorkspaceClient, metastoreId, schemaName string) error {
+	err := w.SystemSchemas.Disable(ctx, catalog.DisableRequest{
+		MetastoreId: metastoreId,
+		SchemaName:  schemaName,
+	})
+	if err != nil {
+		//ignore "<schema-name> system schema can only be disabled by Databricks" error
+		if !strings.Contains(err.Error(), "can only be disabled by Databricks") {
+			return err
+		}
+		log.Printf("[WARN] %s can be disabled only by Databricks, ignoring it", schemaName)
+	}
+	return nil
+}