fix(cookbook): validate model_dir segments in /api/model/cached

[archdex-art]

Summary

GET /api/model/cached validates host and ssh_port (validate_remote_host, validate_ssh_port) but never validated the model_dir query param before splitting it on commas and embedding each segment into the generated cache-scan script — unlike the sibling POST /api/model/download endpoint, which runs its local_dir param through _validate_local_dir. Paths reach the generated script via Python repr() rather than shell interpolation, so this was not a shell-injection RCE, but the inconsistency meant an invalid segment (shell metacharacters, a --prefixed path) could still reach the scanner and trigger an unintended filesystem walk instead of a clean 400. This extracts the split/normalize/validate logic into a new _parse_model_dirs() helper that reuses _validate_local_dir so both endpoints enforce identical rules.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5508

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.

How to Test

1. python -m pytest tests/test_cookbook_helpers.py — 73 passed (5 new tests added for _parse_model_dirs), 1 pre-existing skip.
2. python -m pytest tests/ -k cookbook — 223 passed, 1 pre-existing skip, 1 unrelated pre-existing failure (confirmed via git stash against unmodified dev: test_container_opt_in_with_unix_socket_is_allowed fails with OSError: AF_UNIX path too long, a macOS pytest-tmp-path-length artifact in a Docker-socket test, nothing to do with this change).
3. Live app run: started with AUTH_ENABLED=false ODYSSEUS_DATA_DIR=/tmp/odysseus-data uvicorn app:app --host 127.0.0.1 --port 7001, then hit the real endpoint:
   • curl "http://127.0.0.1:7001/api/model/cached?model_dir=/tmp/odysseus-data" → HTTP 200, real scan results returned.
   • curl "http://127.0.0.1:7001/api/model/cached?model_dir=/tmp/x%3Btouch%20/tmp/pwned" (shell metacharacter) → HTTP 400, {"detail":"Invalid local_dir — must be an absolute or ~ path with no shell metacharacters"} — rejected before reaching the scan script.
   • curl "http://127.0.0.1:7001/api/model/cached?model_dir=/models/-rf" (leading-dash segment / option injection) → HTTP 400, {"detail":"Invalid local_dir — path segments cannot start with '-'"}.
   • curl "http://127.0.0.1:7001/api/model/cached?model_dir=/tmp/odysseus-data,/tmp/odysseus-data" (multi-segment) → HTTP 200, confirms comma-splitting still works for valid paths.
4. python -m py_compile routes/cookbook_helpers.py routes/cookbook_routes.py — passes.

Visual / UI changes — REQUIRED if you touched anything that renders

Not applicable — backend-only change (Python route handler + helper), no rendering code touched.