🐛 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

tsconfig.json

@@ -20,7 +20,8 @@
 		],
 		"paths": {
 			"@/*": ["./src/*"]
-		}
+		},
+		"types": ["bun-types"]
 	},
 	"include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
 	"exclude": ["node_modules"]