🐛 fix: normalize AI-generated dates to valid ISO offset datetimes

- add normalizeAiDateString() to coerce bare ISO datetimes, date-only
  strings, and fractional-second variants into offset-aware format
- apply via z.preprocess in AiEventResponseItemSchema so Zod validation
  no longer rejects AI responses missing a timezone offset
- fix system prompt to use toISOString() (unambiguous UTC) and clarify
  expected datetime format for the AI model
- install bun-types and add to tsconfig so bun:test resolves cleanly
- add 8 behaviour-driven tests covering all normalizer edge cases

package.json

@@ -46,6 +46,7 @@
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "@typescript/native-preview": "^7.0.0-dev.20260407.1",
+    "bun-types": "^1.3.11",
     "drizzle-kit": "^0.31.4",
     "eslint": "^9",
     "eslint-config-next": "15.4.6",