test: strengthen ky things route coverage

[needsbuilder]

Summary

• await and assert the ky create response before listing things
• add ky form-body coverage for the delete route
• add list route coverage for empty state, insertion order, and generated ids

Tests

• bun test
• bun run build
• bun run typecheck
• bun run format:check

/claim #2

[needsbuilder]

포화된 바운티라 철회합니다. 이미 작년에 보상 완료된 이슈인데 제가 놓쳤네요. 🙏