fix(openapi): normalize Path defaults to POSIX in schema generation

[Pfannkuchensack]

Summary

Pydantic stringifies Path defaults via the host OS's separator, so a default like Path("models/.convert_cache") serialized to "models\.convert_cache" whenever the schema was regenerated on Windows — polluting every diff with cross-OS noise.

Added normalize_path_defaults() to walk the finished OpenAPI schema and flip backslashes to forward slashes on any node with format: path. Ran once at the end of get_openapi_func so it covers all current and future Path-typed fields without per-field annotations. Regenerated openapi.json and schema.ts cleanly on Windows.

Related Issues / Discussions

None — this is a developer-experience fix for contributors running make frontend-openapi / pnpm typegen on Windows.

QA Instructions

1. On Linux/macOS: regenerate the schema with make frontend-openapi (or the manual python scripts/generate_openapi_schema.py > invokeai/frontend/web/openapi.json && pnpm prettier --write openapi.json flow). Confirm git diff origin/main -- invokeai/frontend/web/openapi.json invokeai/frontend/web/src/services/api/schema.ts shows no change for any models/.convert_cache / models/.download_cache defaults (they should stay as /).
2. On Windows (PowerShell): from invokeai/frontend/web/ run

   uv run --extra cuda python ../../../scripts/generate_openapi_schema.py | pnpm typegen
   

   Then git diff -- src/services/api/schema.ts | grep "models\\\\" — should return nothing. Without this fix you'd see lines like @default models\.convert_cache.
3. Grep the regenerated artifacts for any leftover backslash drift on path defaults:

   grep -E "@default models\\\\" invokeai/frontend/web/src/services/api/schema.ts
   grep -E "\"default\": \"models\\\\\\\\" invokeai/frontend/web/openapi.json
   

   Both should return no matches on any OS.
4. The intentional Windows-path example inside the install_model docstring (C:\\users\\fred\\model.safetensors) is unaffected because it lives in a description, not a default, and isn't on a node with format: path. Confirm it's still present in openapi.json line ~2818.
5. App startup unchanged — the normalization runs only at schema-generation time inside get_openapi_func.

Merge Plan

No coordination needed. Pure dev-tooling fix; runtime behaviour is unchanged. After merge, anyone who currently has Windows-flavoured schema drift in their branch can rebase + regen cleanly.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) — pure schema-serialization helper; covered by manual regen verification above
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)