test: assert inventory rejection rollback

[JOY (JOY)]

Summary

• add post-error no-mutation assertions for rejected inventory use, salvage, and stash move paths
• update docs and changelog to reflect the broader Add inventory lifecycle validation tests and guards #219 rollback 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, validates server-side rejection behavior without changing runtime authority or Unity assets.

Completes the post-error no-mutation acceptance coverage in #219.