fix(forecast,docs): address CodeRabbit feedback on PR #266 (3 quick wins)

[w7-mgfcode]

Three quick-win CodeRabbit comments from PR #266 (dev → main release prep for v0.2.17). All flagged in code/docs that already landed on dev. Bundling them under one tracking issue + fix branch to keep the release window clean.

Items

1. app/features/forecasting/service.py:809-812 (CodeRabbit #3270767550) — bundle_path_str is read from job.result["model_path"] (untyped JSONB) and only checked for truthiness. Strengthen to isinstance(str) and stripped non-empty, raising UnprocessableEntityError on type/empty failure.
2. PRPs/PRP-31-mlzoo-d-frontend-registry-explainability.md:17-18 (CodeRabbit #3270767557) — Scope statement says "one new backend endpoint" but the PRP defines two (/runs/{run_id}/feature-metadata, /jobs/{job_id}/feature-metadata). Rephrase to match.
3. PRPs/PRP-31-mlzoo-d-frontend-registry-explainability.md:1210 (CodeRabbit #3270767563) — Route docstring lists `500 — bundle file missing on disk`; the rest of the PRP and the implementation (forecasting/service.py:832-839) use 422. Update to `422 — bundle file missing on disk`.

Out of scope

• The `ModelFamily` slice-isolation refactor (CodeRabbit #3270767554) is a heavy-lift refactor needing its own PRP — tracked in a separate issue.

Approach

• Branch: `fix/coderabbit-pr266-quickwins` off `dev`
• 3 commits, one per fix, all referencing this issue
• Validation gates: ruff, mypy, pyright, pytest on forecasting tests
• PR into `dev` after PR release: prepare v0.2.17 — MLZOO-D + dogfood automation + cross-slice import pattern #266 merges to `main` (avoid resetting the release-prep gate)

Refs: PR #266

[w7-mgfcode]

Closed by #272 (merged 2026-05-20). 3 commits on dev: 79db58f + 1ea196f + 655007a. All 4 CodeRabbit replies posted and threads resolvable by the maintainer.