FIX false-success: opnsense-plugins.sh apply ALWAYS dry-ran (DRY_RUN:+ expands on 0) ...

Found on the first-ever live run (vr1-dc0 edge, 2026-07-20): ${DRY_RUN:+--dry-run}
expands whenever the var is set and NON-EMPTY, and DRY_RUN=0 is non-empty -- so
every apply silently dry-ran while printing 'OK: profile applied'. Nothing was
ever installed by this script. The harness missed it because every test drove
--dry-run, leaving the live path with zero coverage. Fix tests the VALUE; harness
gains two live-path tests via a stub api client (verified to FAIL 2/2 against the
buggy version before restoring). 20/20.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KiUu1oqt76tWvV4vEC3NAr