feat: add embedded testing server for postgres (#9955)

# Which Problems Are Solved

1. there was no embedded database to run tests against
2. there were no tests for postgres/migrate
3. there was no test setup for repository which starts a client for the
embedded database

# How the Problems Are Solved

1. postgres/embedded package was added
2. tests were added
3. TestMain was added incl. an example test

# Additional Changes

none

# Additional Context

closes #9934

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/v3/storage/database/dialect/postgres/embedded/start.go

@@ -0,0 +1,50 @@
+// embedded is used for testing purposes
+package embedded
+
+import (
+	"net"
+	"os"
+
+	embeddedpostgres "github.com/fergusstrange/embedded-postgres"
+	"github.com/zitadel/logging"
+
+	"github.com/zitadel/zitadel/backend/v3/storage/database"
+	"github.com/zitadel/zitadel/backend/v3/storage/database/dialect/postgres"
+)
+
+// StartEmbedded starts an embedded postgres v16 instance and returns a database connector and a stop function
+// the database is started on a random port and data are stored in a temporary directory
+// its used for testing purposes only
+func StartEmbedded() (connector database.Connector, stop func(), err error) {
+	path, err := os.MkdirTemp("", "zitadel-embedded-postgres-*")
+	logging.OnError(err).Fatal("unable to create temp dir")
+
+	port, close := getPort()
+
+	config := embeddedpostgres.DefaultConfig().Version(embeddedpostgres.V16).Port(uint32(port)).RuntimePath(path)
+	embedded := embeddedpostgres.NewDatabase(config)
+
+	close()
+	err = embedded.Start()
+	logging.OnError(err).Fatal("unable to start db")
+
+	connector, err = postgres.DecodeConfig(config.GetConnectionURL())
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return connector, func() {
+		logging.OnError(embedded.Stop()).Error("unable to stop db")
+	}, nil
+}
+
+// getPort returns a free port and locks it until close is called
+func getPort() (port uint16, close func()) {
+	l, err := net.Listen("tcp", ":0")
+	logging.OnError(err).Fatal("unable to get port")
+	port = uint16(l.Addr().(*net.TCPAddr).Port)
+	logging.WithFields("port", port).Info("Port is available")
+	return port, func() {
+		logging.OnError(l.Close()).Error("unable to close port listener")
+	}
+}