fix(schedule): reject cron expressions that never match

Since cron-parser 5.7, an expression such as `0 0 31 4,6 *` parses but
throws from next() because no date matches. Validation only parsed, so
the raw parser error escaped from run() instead of
E_INVALID_CRON_EXPRESSION. Ask for one occurrence while validating.

Author: RomainLanz

src/schedule_builder.ts

@@ -148,10 +148,11 @@ export class ScheduleBuilder<TPayload = unknown> implements PromiseLike<Schedule
       ])
     }
 
-    // Validate cron expression
+    // Validate cron expression. Some expressions parse but never match
+    // (`0 0 31 4,6 *`), and only throw once asked for an occurrence.
     if (this.#cronExpression) {
       try {
-        CronExpressionParser.parse(this.#cronExpression, { tz: this.#timezone })
+        CronExpressionParser.parse(this.#cronExpression, { tz: this.#timezone }).next()
       } catch (error) {
         throw new errors.E_INVALID_CRON_EXPRESSION(
           [this.#cronExpression, (error as Error).message],

tests/schedule.spec.ts

@@ -227,6 +227,14 @@ test.group('ScheduleBuilder', (group) => {
     }
   })
 
+  test('should throw for a cron expression that never matches', async ({ assert }) => {
+    // Parses fine, but April and June have no 31st
+    await assert.rejects(
+      () => new ScheduleBuilder('NeverJob', {}).cron('0 0 31 4,6 *').run(),
+      errors.E_INVALID_CRON_EXPRESSION
+    )
+  })
+
   test('should be thenable for auto-run', async ({ assert }) => {
     const builder = new ScheduleBuilder('SyncJob', { source: 'api' })