[scheduler] Validate the FREQ value in parseRRule

`parseRRule` cast the RRULE-string `FREQ` straight to the union without
checking it, and the object-input branch validated nothing. An unsupported
value like `FREQ=HOURLY` passed parsing and failed later with a message
disconnected from the input.

Validate `freq` against DAILY/WEEKLY/MONTHLY/YEARLY in both branches and
throw a clear `MUI X Scheduler:` error naming the offending value.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rita-codes

docs/public/static/error-codes.json

@@ -283,5 +283,6 @@
   "283": "MUI X Scheduler: TitleColumnWidthProvider is missing.",
   "284": "MUI X Data Grid: Nested lazy loading does not support unknown children count for now.\nIf this is a use-case that you are interested in, please open an issue: https://github.com/mui/mui-x/issues/new?template=2.feature.yml",
   "285": "MUI X Data Grid: Row count is unknown. Please provide a valid row count for lazy loading to work.",
-  "286": "MUI X Scheduler: useSharedComponentsStyledContext must be used within a SharedComponentsStyledContext.Provider. The shared internal components require the product to inject their utility classes. Ensure the component is rendered inside an EventCalendar, EventCalendarPremium, or EventTimelinePremium component, a standalone view, or another component that provides SharedComponentsStyledContext."
+  "286": "MUI X Scheduler: useSharedComponentsStyledContext must be used within a SharedComponentsStyledContext.Provider. The shared internal components require the product to inject their utility classes. Ensure the component is rendered inside an EventCalendar, EventCalendarPremium, or EventTimelinePremium component, a standalone view, or another component that provides SharedComponentsStyledContext.",
+  "287": "MUI X Scheduler: Invalid FREQ value \"%s\". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value."
 }

packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.test.ts

@@ -115,6 +115,20 @@ describe('recurring-events/rRuleString', () => {
       );
     });
 
+    it('should throw when FREQ has an unsupported value', () => {
+      expect(() => parseRRule(adapter, 'FREQ=HOURLY', 'default')).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "HOURLY". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
+    it('should throw when the object input has an unsupported freq value', () => {
+      expect(() =>
+        parseRRule(adapter, { freq: 'HOURLY' as SchedulerEventRecurrenceRule['freq'] }, 'default'),
+      ).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "HOURLY". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
     it('should throw when the RRULE contains unsupported properties', () => {
       expect(() => parseRRule(adapter, 'FREQ=DAILY;FOO=bar', 'default')).to.throw(
         'MUI X Scheduler: Unsupported RRULE property "FOO". Supported properties are: FREQ, INTERVAL, BYDAY, BYMONTHDAY, BYMONTH, UNTIL, COUNT. Remove or replace the unsupported property.',

packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.ts

@@ -18,12 +18,31 @@ const SUPPORTED_RRULE_KEYS = new Set([
   'COUNT',
 ]);
 
+const SUPPORTED_FREQUENCIES = new Set<SchedulerProcessedEventRecurrenceRule['freq']>([
+  'DAILY',
+  'WEEKLY',
+  'MONTHLY',
+  'YEARLY',
+]);
+
+function validateFreq(freq: string): SchedulerProcessedEventRecurrenceRule['freq'] {
+  if (!SUPPORTED_FREQUENCIES.has(freq as SchedulerProcessedEventRecurrenceRule['freq'])) {
+    throw new Error(
+      `MUI X Scheduler: Invalid FREQ value "${freq}". ` +
+        'The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. ' +
+        'Provide a supported frequency value.',
+    );
+  }
+  return freq as SchedulerProcessedEventRecurrenceRule['freq'];
+}
+
 export function parseRRule(
   adapter: Adapter,
   input: string | SchedulerEventRecurrenceRule,
   timezone: TemporalTimezone,
 ): SchedulerProcessedEventRecurrenceRule {
   if (typeof input === 'object') {
+    validateFreq(input.freq);
     if (input.until != null) {
       return {
         ...input,
@@ -72,7 +91,7 @@ export function parseRRule(
   }
 
   const rrule: SchedulerProcessedEventRecurrenceRule = {
-    freq: rruleObject.FREQ as SchedulerProcessedEventRecurrenceRule['freq'],
+    freq: validateFreq(rruleObject.FREQ),
   };
 
   if (rruleObject.INTERVAL) {