test: cover inventory validation rejections

[JOY (JOY)]

Summary

• add inventory rejection coverage for same-slot replacement, invalid slot equip, and full body carry capacity
• assert rejected inventory paths leave the storage object unchanged
• assert a DOS.AI/model inventory mutation intent is denied by server validation and does not mutate inventory
• update the inventory design checklist and changelog for the completed Add inventory lifecycle validation tests and guards #219 validation coverage

Verification

• npm.cmd run build in backend/nakama
• npm.cmd test in backend/nakama
• npx.cmd markdownlint-cli2 "CHANGELOG.md" "docs/design/45-inventory-and-equipment-system.md"
• git diff --check

Local review fallback

• APPROVED: test/docs-only backend slice, no Unity package/scene changes, no client-side keys, and LLM/model output remains non-authoritative.

Completes the explicit validation-test checklist item in #219. Broader custody lifecycle state expansion remains tracked separately in the same issue.