Kubernetes secret store does not declare FeatureMultipleKeyValuesPerSecret despite supporting it #4544

Description

The Kubernetes secret store's Features() returns an empty list:

// secretstores/kubernetes/kubernetes.go
func (k *kubernetesSecretStore) Features() []secretstores.Feature {
	return []secretstores.Feature{}
}

But its own GetSecret()/BulkGetSecret() implementations already correctly return multiple key-value pairs per secret:

func (k *kubernetesSecretStore) GetSecret(ctx context.Context, req secretstores.GetSecretRequest) (secretstores.GetSecretResponse, error) {
	resp := secretstores.GetSecretResponse{Data: map[string]string{}}
	...
	for k, v := range secret.Data {
		resp.Data[k] = string(v)
	}
	return resp, nil
}

This is correct behavior, since a Kubernetes Secret resource's Data field is inherently a map[string][]byte - multiple keys per secret is the normal case, not an edge case.

Comparison across all secret store providers

I checked Features() across every provider in secretstores/:

• Correctly declare FeatureMultipleKeyValuesPerSecret (multi-key-value backend): local/file, hashicorp/vault (conditionally, based on vaultValueType), aws/secretmanager
• Correctly declare no features (genuinely single-value-per-secret backend): azure/keyvault, gcp/secretmanager, huaweicloud/csms, tencentcloud/ssm - each has an explicit // No Feature supported. comment, consistent with their backend's actual data model
• kubernetes: the one outlier - same multi-key-value backend shape as the first group, but declares no features

Impact

I couldn't find FeatureMultipleKeyValuesPerSecret wired into any runtime control-flow decision in dapr/dapr today (only referenced in test mocks), so this doesn't currently break request handling. It is, however, a real metadata/capability-advertisement inconsistency: any tooling, SDK, or dashboard that inspects a secret store's declared features to decide how to present or handle its results would get an inaccurate answer for the Kubernetes provider specifically.

Proposed fix

func (k *kubernetesSecretStore) Features() []secretstores.Feature {
	return []secretstores.Feature{secretstores.FeatureMultipleKeyValuesPerSecret}
}

Happy to open a PR with this plus a regression test asserting the declared feature matches the actual multi-key return behavior (similar in spirit to the existing Vault tests, which cover the conditional case).