- 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
