fix(dashboard): clean error when model record lacks gguf_file - #2920

Summary

A persisted model-library record with an id but no gguf_file raised an
unhandled KeyError on download/delete, surfacing as a 500. These paths
now read gguf_file via .get() and return a clean 404 when it is missing,
matching the sibling .get()-based access used elsewhere in the module.

AI Assistance

AI-assisted draft; human reviewed the diff and chose the validation below.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Dashboard API / host agent
• Installer / bootstrap / lifecycle
• Docker Compose / service manifests
• Model routing / Hermes / capabilities
• Network exposure / auth / proxy
• Dependencies / runtime wiring

Risk And Validation

• Risk level: Low
• Validation run:
  • git diff --check
  • Not required because: narrow change; syntax and diff checks pass locally

Commands/results:

git diff --check clean; bash -n on touched scripts passes

Operational Change Check

• This is an operational change and validation is recorded above.