🐛 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

tests/date-normalizer.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, test } from "bun:test";
+import { normalizeAiDateString } from "@/lib/date-normalizer";
+import { AiEventResponseSchema } from "@/lib/types";
+
+describe("normalizeAiDateString", () => {
+	test("converts bare ISO datetime (no offset) to valid offset datetime accepted by Zod", () => {
+		const input = "2025-04-10T10:00:00";
+		const result = normalizeAiDateString(input);
+		expect(result).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2})$/);
+		expect(new Date(result).getHours()).toBe(10);
+	});
+
+	test("converts date-only string to midnight datetime with offset", () => {
+		const input = "2025-04-10";
+		const result = normalizeAiDateString(input);
+		expect(result).toMatch(/^\d{4}-\d{2}-\d{2}T00:00:00(Z|[+-]\d{2}:\d{2})$/);
+	});
+
+	test("passes through already-valid datetime with offset unchanged", () => {
+		const input = "2025-04-10T10:00:00-04:00";
+		const result = normalizeAiDateString(input);
+		expect(result).toBe("2025-04-10T10:00:00-04:00");
+	});
+
+	test("passes through datetime with Z suffix unchanged", () => {
+		const input = "2025-04-10T10:00:00Z";
+		const result = normalizeAiDateString(input);
+		expect(result).toBe("2025-04-10T10:00:00Z");
+	});
+
+	test("handles datetime with fractional seconds", () => {
+		const input = "2025-04-10T10:00:00.000Z";
+		const result = normalizeAiDateString(input);
+		expect(result).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/);
+		expect(new Date(result).getHours()).toBe(10);
+	});
+});
+
+describe("AiEventResponseSchema date normalization", () => {
+	test("accepts event with bare ISO datetime (no offset) by normalizing it", () => {
+		const raw = [
+			{
+				title: "Team Meeting",
+				start: "2025-04-10T10:00:00",
+				end: "2025-04-10T11:00:00",
+			},
+		];
+		const result = AiEventResponseSchema.safeParse(raw);
+		expect(result.success).toBe(true);
+		if (result.success) {
+			expect(result.data[0].start).toMatch(
+				/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2})$/,
+			);
+		}
+	});
+
+	test("accepts event with date-only start and no end", () => {
+		const raw = [
+			{
+				title: "All-day workshop",
+				start: "2025-07-20",
+				allDay: true,
+			},
+		];
+		const result = AiEventResponseSchema.safeParse(raw);
+		expect(result.success).toBe(true);
+	});
+
+	test("accepts event with already-valid offset datetime", () => {
+		const raw = [
+			{
+				title: "Standup",
+				start: "2025-04-10T09:00:00-05:00",
+				end: "2025-04-10T09:30:00-05:00",
+			},
+		];
+		const result = AiEventResponseSchema.safeParse(raw);
+		expect(result.success).toBe(true);
+		if (result.success) {
+			expect(result.data[0].start).toBe("2025-04-10T09:00:00-05:00");
+		}
+	});
+});