test: strengthen ky route coverage

[caaeco]

Summary

• await and validate the ky-backed create request before listing records
• assert the persisted record fields and generated ID
• add end-to-end coverage for ky URL-encoded form submission to the delete route
• make the delete test resilient to concurrently created records

Validation

• bun test - 3 passed, 0 failed
• winterspec bundle -o dist/bundle.js - passed
• tsc --noEmit - passed
• biome format tests/routes/things/create.test.ts tests/routes/things/delete.test.ts - passed

/claim #2