fix(config): validate volume mount paths syntactically instead of filepath

[spapa013]

Stacked on nauticalab#24

Summary

• replace filepath validation on VolumeMount.localPath and VolumeMount.containerPath with mount_path
• implement mount_path as syntax-only validation (absolute path shape), with no filesystem checks
• add regression tests to verify:
  • /mnt is accepted
  • /mnt/data is accepted
  • empty mount paths are rejected

Problem

filepath validation is filesystem-dependent and file-oriented (os.Stat-based), so valid directory mount roots like /mnt were being rejected during devenv generate depending on runtime environment.

Validation

• automated: go test -v -race -covermode=atomic -coverprofile=./coverage.out ./... (pass)
• manual: generated config, SSH’d into pod, and confirmed /mnt mounted as expected