[Batch] Template and profile registry never called #2626

🐛 Describe the bug

_load_batch_k8s_context in aibrix/metadata/app.py builds the CoreV1Api/AppsV1Api k8s clients under --enable-k8s-support but never instantiates the TemplateRegistry or ProfileRegistry from the fixed-name ConfigMaps, so both stay None. Any batch that references a model template by name (e.g. {"model_template": {"name": "mock-vllm"}}, no inline spec) then fails in JobManifestRenderer with RenderError("template registry is not configured"), surfaced as inference_failed on the batch.

Steps to Reproduce

1. Install via Helm/VKE instructions and apply samples/batch/batch_v1alpha1_{batch_profiles,model_deployment_templates}.yaml.
2. Submit a batch with aibrix.model_template.name = "mock-vllm" / profile.name = "prod-24h" and runtime.target = "KubernetesJob" (no inline spec), referencing only the ConfigMap entries by name.
3. Poll the batch. Expected before fix: errors.data[0].message == "template registry is not configured".

Expected behavior

MDS startup, under --enable-k8s-support, reads POD_NAMESPACE, builds the two registries and loads before serving.

Environment

AIBrix version: released v0.7.0
Development environment: Kubernetes