test: improve error featureGates precheck messages

[yaroslavborbat]

Description

Improve feature gate precheck errors: print ready kubectl patch command for missing gates; treat absent featureGates as empty list.

Why do we need it, and what problem does it solve?

Hotplug precheck failures did not show how to enable the required feature gate.

What is the expected result?

Precheck error includes a command to enable the gate in ModuleConfig.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: test
type: fix
summary: "E2E feature gate precheck errors now include kubectl patch command to enable missing gates."
impact_level: low