testserver: match cloud API behavior for database_catalogs (#4440)

## Changes
- Assign uid in CREATE response
- Do not return create_database_if_not_exists in GET (write-only field)

## Why

This causes the migrate test to exercise the code path where
database_catalogs has an Update action (due to the field difference),
which triggered a nil pointer panic before commit
a0418ac.

## Tests
Revert #4438 - can now see panic
with local testserver.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: denik

libs/testserver/database_catalogs.go

@@ -34,13 +34,34 @@ func (s *FakeWorkspace) DatabaseCatalogCreate(req Request) Response {
 		}
 	}
 
+	// Assign uid like the real backend does
+	databaseCatalog.Uid = nextUUID()
+
 	s.DatabaseCatalogs[databaseCatalog.Name] = databaseCatalog
 
 	return Response{
 		Body: databaseCatalog,
 	}
 }
 
+func (s *FakeWorkspace) DatabaseCatalogGet(name string) Response {
+	defer s.LockUnlock()()
+
+	catalog, ok := s.DatabaseCatalogs[name]
+	if !ok {
+		return Response{
+			Body:       fmt.Sprintf("database catalog with name '%s' not found", name),
+			StatusCode: 404,
+		}
+	}
+
+	// create_database_if_not_exists is a write-only field - the real API doesn't return it
+	result := catalog
+	result.CreateDatabaseIfNotExists = false
+
+	return Response{Body: result}
+}
+
 func (s *FakeWorkspace) DatabaseCatalogUpdate(req Request, name string) Response {
 	defer s.LockUnlock()()
 

libs/testserver/handlers.go

@@ -607,7 +607,7 @@ func AddDefaultHandlers(server *Server) {
 	})
 
 	server.Handle("GET", "/api/2.0/database/catalogs/{name}", func(req Request) any {
-		return MapGet(req.Workspace, req.Workspace.DatabaseCatalogs, req.Vars["name"])
+		return req.Workspace.DatabaseCatalogGet(req.Vars["name"])
 	})
 
 	server.Handle("PATCH", "/api/2.0/database/catalogs/{name}", func(req Request) any {