refactor: extract get_public_ip into shared helpers.sh

[Kewe63]

Summary

Centralizes public IP detection logic into a shared helper script.

Problem

Identical 23-line implementations of get_public_ip() were duplicated in multiple entrypoints, violating DRY principles.

Solution

• Created helpers.sh with a single validated implementation.
• Updated entrypoints to source ./helpers.sh.
• Added error handling for curl failures under set -e.

Verification

• Verified functionality remains identical.

Impact

Reduces maintenance burden.

[cb-heimdall]

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