fix(cookbook): remove dead IS_WINDOWS import in _pip_install_fallback_chain

[archdex-art]

Summary

_pip_install_fallback_chain (routes/cookbook_helpers.py) imports IS_WINDOWS from core.platform_compat at the top of the function but never references it anywhere in the function body — dead code left over from an earlier version of the pip install fallback chain. This removes the unused import. No behavior change.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5512

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. grep -c IS_WINDOWS routes/cookbook_helpers.py — returns 0 (no remaining references anywhere in the file).
2. python -m py_compile routes/cookbook_helpers.py — passes.
3. python -m pytest tests/test_cookbook_helpers.py — 68 passed, 1 pre-existing skip, same as before the change.
4. Live app run: AUTH_ENABLED=false ODYSSEUS_DATA_DIR=/tmp/odysseus-data uvicorn app:app --host 127.0.0.1 --port 7002, confirmed clean startup with no ImportError/traceback in logs, then curl http://127.0.0.1:7002/api/model/cached returns HTTP 200 — proves routes/cookbook_helpers.py (which _pip_install_fallback_chain lives in) imports and the route registers cleanly with the import removed.

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

Not applicable — this change touches no rendering code (backend Python only, one dead import line removed).