helper: Bound internet checks so a DNS hang is not INTERNET_CHECK_FAIL

[joaoantoniocardoso]

Summary

• Helper waited for every website probe, including DNS lookups that ignore the 5s socket timeout. That latency matches the frontend's 10s Axios deadline, so a single lost UDP query to 8.8.8.8 showed INTERNET_CHECK_FAIL while ICMP still worked.
• Internet checks now return within 4s (partial results if a name is still resolving), run off the event loop, and use HEAD. Service scans skip Zenoh 7117/7447. In-flight temporary_cache calls coalesce. The UI keeps the last verdict until three polls in a row fail.

Fixes #4176

Test plan

• Open BlueOS with working internet: globe tooltip is online, notifications stay empty
• GET /helper/latest/check_internet_access returns in well under 10s even if one hostname is slow to resolve
• Helper logs no longer show 1s timeouts on Zenoh 7447
• A single timed-out poll does not clear has_internet or raise INTERNET_CHECK_FAIL