agent-platform release v4.28.2

The serving slice renders without an edge Gateway (giantswarm/agent-platform#490): connectivity’s validateIngress required a public Gateway for the muster / route and a muster-direct-consistent components.agentgateway toggle in every release, muster on or off — so the <cluster>-agent-platform slice (examples/serving-slice.yaml: no muster, its own models Gateway; agentgateway on where the target runs no controller) failed its render on gazelle with no public Gateway for ingress.parentRefs. Both checks now apply only while components.muster is on; the mode enum, the agentgateway-direct refusal, the “agentgateway-* mode needs the component” rule and the per-component muster-direct guards are unchanged. tests/verify-serving-slice.py renders the slice without any ingress.parentRefs and holds the guard for a release with muster on.