minReplicas cannot be set to 0 when using KEDA ScaledObject

[ArtemGontar]

Describe the feature

Flagger validates spec.autoscalerRef.primaryScalerReplicas.minReplicas to be >= 1, but when using KEDA as an autoscaler, minReplicaCount: 0 is a valid and expected configuration.

This makes it impossible to use Flagger with KEDA scale-to-zero setups.

Error message
Error: 1 error occurred:
    * Canary.flagger.app is invalid:
      spec.autoscalerRef.primaryScalerReplicas.minReplicas:
      Invalid value: 0:
      spec.autoscalerRef.primaryScalerReplicas.minReplicas in body should be greater than or equal to 1

Expected behavior

When autoscalerRef points to a KEDA ScaledObject, Flagger should allow minReplicas: 0

This matches KEDA behavior and is required for event-driven / scale-to-zero workloads.

Actual behavior

Flagger enforces minReplicas >= 1, causing Canary creation to fail when minReplicas is set to 0.

Reproduction steps

Use KEDA with a ScaledObject that allows minReplicaCount: 0

Reference it in a Flagger Canary via autoscalerRef

Set primaryScalerReplicas.minReplicas: 0

Apply the Canary

Validation fails with the error above

Canary configuration (relevant part)

autoscalerRef:
  apiVersion: keda.sh/v1alpha1
  kind: ScaledObject
  name: my-app
  primaryScalerReplicas:
    minReplicas: 0
    maxReplicas: 10

Helm template example

This is a common pattern when supporting both critical workloads and KEDA scale-to-zero:

autoscalerRef:
  apiVersion: keda.sh/v1alpha1
  kind: ScaledObject
  name: {{ .Values.name }}
  primaryScalerReplicas:
    {{- if and .Values.critical.enabled (lt .Values.autoscaling.minReplicas 2.0) }}
    minReplicas: 2
    {{- else }}
    minReplicas: {{ .Values.autoscaling.minReplicas }}
    {{- end }}
    maxReplicas: {{ .Values.autoscaling.maxReplicas }}

When .Values.autoscaling.minReplicas = 0, this fails validation even though KEDA supports it.