refactor: make fetchContext required on FeedSource

Sources that cannot provide context now return null
instead of omitting the method. The engine checks the
return value rather than method existence.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-source-apple-calendar/src/calendar-source.ts

@@ -58,7 +58,7 @@ export class CalendarSource implements FeedSource<CalendarFeedItem> {
 		this.injectedClient = options?.davClient ?? null
 	}
 
-	async fetchContext(context: Context): Promise<Partial<Context>> {
+	async fetchContext(context: Context): Promise<Partial<Context> | null> {
 		const events = await this.fetchEvents(context)
 		if (events.length === 0) {
 			return {