🐛 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

src/app/api/ai-event/route.ts

@@ -17,7 +17,7 @@ TypeScript type:
   description?: string,
   location?: string,
   url?: string,
-  start: string, // ISO datetime
+  start: string, // ISO 8601 datetime with timezone, e.g. "2025-04-10T10:00:00-04:00"
   end?: string,
   allDay?: boolean,
   recurrenceRule?: string // valid iCal RRULE string like FREQ=WEEKLY;BYDAY=MO;INTERVAL=1
@@ -26,7 +26,7 @@ TypeScript type:
 Rules:
   - If the user describes multiple events in one prompt, return multiple objects (one per event).
   - Always return a valid JSON array of objects, even if there's only one event.
-  - Today is ${new Date().toLocaleString()}.
+  - Today is ${new Date().toISOString()}.
   - If no time is given, assume allDay event.
   - If no end time is given (and event is not allDay), default to 1 hour after start.
   - If multiple events are described, return multiple.