fix(localdns): reduce startup polling interval

[jingwenw15]

Summary

• reduce localdns PID-file polling from 1s to 0.1s while preserving the existing timeout semantics
• reduce localdns readiness polling from 1s to 0.1s and scale the production call-site attempts accordingly
• add focused ShellSpec coverage for the 0.1-second polling interval in both helper loops

Testing

• shellspec --shell bash spec/parts/linux/cloud-init/artifacts/localdns_spec.sh:668
• shellspec --shell bash spec/parts/linux/cloud-init/artifacts/localdns_spec.sh:733

[Copilot]

Pull request overview

Reduces localdns startup polling intervals from 1s to 0.1s so the node bootstrap path waits less when CoreDNS comes up quickly, while preserving the existing wall-clock timeout semantics. The PID-file wait now counts in tenths of a second (10× the timeout), and the readiness call-site bumps maxattempts from 60 to 600 to match the 60s timeout at the finer interval.

Changes:

• Introduce LOCALDNS_POLL_INTERVAL_SECONDS=0.1 and rework start_localdns to use tenth-second counters so START_LOCALDNS_TIMEOUT=10 still yields a ~10s ceiling.
• Switch wait_for_localdns_ready's sleep to 0.1s and update the production call-site to wait_for_localdns_ready 600 60.
• Add ShellSpec tests asserting both loops sleep with 0.1.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
parts/linux/cloud-init/artifacts/localdns.sh	Faster polling in start_localdns/wait_for_localdns_ready plus updated max-attempt arg at the call-site.
spec/parts/linux/cloud-init/artifacts/localdns_spec.sh	New ShellSpec cases asserting the 0.1s sleep interval in both helper loops.