Validation: allow mounting the same volume at multiple paths

Currently, `Container.volume_mounts` in `ateapi.proto` is defined with `+k8s:listMapKey=name`. This enforces that each volume name appears at most once per container, disallowing a single volume from being mounted at more than one path.

In standard Kubernetes, mounting the same volume to multiple paths within a container is fully supported and idiomatic (with uniqueness enforced on mountPath, not name).

Proposal - Update `Container.volume_mounts` in `pkg/proto/ateapipb/ateapi.proto` to use `+k8s:listMapKey=mount_path`.
Add E2E tests verifying multi-mount behavior across read-only image volumes, writable durable-dir volumes, and external (CSI) volumes, ensuring shared writes and mounts remain consistent across actor suspend/resume cycles.

cc: @msau42 @juli4n