REGTESTS: protobuf: add regression test for nested vs flat paths

REGTESTS: protobuf: add regression test for nested vs flat paths

This patch adds a new VTC regression test to validate the protobuf()
sample converter and prevent future regressions.

It tests four distinct scenarios to ensure correct behavior:
  - validating legitimate nested path extraction (e.g., path "1.2") on a
    nested payload.
  - ensuring flat path extraction (path "2") is blocked when the field is
    actually inside a nested payload.
  - validating legitimate flat sibling extraction (path "2") on a flat
    payload.
  - ensuring nested path extraction (path "1.2") is blocked when run on
    a flat sibling payload which was fixed by this commit

    BUG/MEDIUM: protobuf: fix nested path bypass in field lookup