Harden Debian 12/13 builder scripts for CI reliability

[Copilot]

Debian builder scripts lacked the error-handling guarantees present in the Rocky Linux builders, making CI failures silent and non-deterministic.

Changes

• pkg/debian/builder-debian{12,13}/builder.sh
  • Added set -euo pipefail — fail fast on errors, unset variables, and broken pipes (already present in Rocky builders)
  • Pinned fpm to 1.17.0 with --no-document for reproducible, faster gem installs (matches Rocky builders)
  • Changed ${STRING_PARAM} → ${STRING_PARAM:-} — prevents unbound variable abort under set -u when STRING_PARAM is not exported into the container

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