🐛 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/lib/types.ts

@@ -1,5 +1,6 @@
 import { z } from "zod";
 import { MAX_IMAGE_SIZE_BYTES } from "@/lib/constants";
+import { normalizeAiDateString } from "@/lib/date-normalizer";
 
 /** Validates that a base64 data URL string decodes to binary under the max size. */
 const isValidImageSize = (val: string | undefined): boolean => {
@@ -29,14 +30,19 @@ export const AiEventRequestSchema = z
 
 export type AiEventRequest = z.infer<typeof AiEventRequestSchema>;
 
+const aiDatetime = z.preprocess(
+	(val) => (typeof val === "string" ? normalizeAiDateString(val) : val),
+	z.string().datetime({ offset: true }),
+);
+
 export const AiEventResponseItemSchema = z.object({
 	id: z.string().optional(),
 	title: z.string().min(1),
 	description: z.string().optional(),
 	location: z.string().optional(),
 	url: z.string().optional(),
-	start: z.string().datetime({ offset: true }),
-	end: z.string().datetime({ offset: true }).optional(),
+	start: aiDatetime,
+	end: aiDatetime.optional(),
 	allDay: z.boolean().optional(),
 	recurrenceRule: z.string().optional(),
 });