Harden model factory handling of optional PrintNode fields

[cbusillo]

Problem

ModelFactory still assumes some nested PrintNode response keys always exist. Examples include create_computer expecting jre / inet_6, and create_printer expecting nested computer / capabilities keys. Current tests pin the happy-path behavior, but API payloads can drift or omit optional fields.

Suggested work

• Add credential-free regression tests for missing optional fields and null nested values.
• Decide which omissions should be accepted with defaults versus rejected clearly.
• Prefer small compatibility-preserving changes over a broad model redesign.
• Update CHANGELOG.md for any behavior users may notice.

Verification

Run uv run pytest and, if model behavior changes, include a clean wheel build/Twine check.