fix(network_preflight): guard skipped fallback DNS probe result

[joshavant]

Summary

• avoid selecting network_preflight_dns_resolution_after_fallback when the fallback probe task was skipped
• require a real fallback probe result (rc present) before using it as the effective DNS probe
• add a regression assertion in logic tests to keep this guard in place

Root Cause

network_preflight_dns_resolution_after_fallback is still defined when its task is skipped, but that skipped result does not include rc. The previous selector treated "defined" as sufficient, and later conditionals accessed .rc, causing provisioning to fail during DNS preflight.

Testing

• python3 -m pytest -q tests/logic_py/test_docs.py -k network_preflight
• python3 -m pytest -q tests/logic_py