fix(emails): send METHOD:CANCEL in ICS for cancelled bookings (#29708)

Fixes an issue where cancellation emails were sending ICS files with
METHOD:REQUEST instead of METHOD:CANCEL. As per RFC 5546, calendar
clients only process cancellations correctly when METHOD:CANCEL is used,
otherwise cancelled events remain in attendee calendars with active
reminders.

- Use METHOD:CANCEL for cancelled booking ICS files
- Keep METHOD:REQUEST for non-cancelled booking invitations
- Preserve STATUS:CANCELLED and sequence handling
- Add tests to verify the generated ICS method for cancellations

packages/emails/lib/generateIcsFile.ts

@@ -36,6 +36,6 @@ export default function generateIcsFile({
       status,
       t,
     }),
-    method: "REQUEST",
+    method: status === "CANCELLED" ? "CANCEL" : "REQUEST",
   };
 }

packages/emails/lib/generateIcsString.test.ts

@@ -46,6 +46,9 @@ const testIcsStringContains = ({
   }
   expect(icsString).toEqual(expect.stringContaining(`DTEND:${DTEND}`));
   expect(icsString).toEqual(expect.stringContaining(`STATUS:${status}`));
+  expect(icsString).toEqual(
+    expect.stringContaining(`METHOD:${status === "CANCELLED" ? "CANCEL" : "REQUEST"}`)
+  );
   //   Getting an error expected icsString to deeply equal stringMatching
   //   for (const attendee of event.attendees) {
   //     expect(icsString).toEqual(

packages/emails/lib/generateIcsString.ts

@@ -61,7 +61,7 @@ const generateIcsString = ({
   const location = getVideoCallUrlFromCalEvent(event) || event.location;
 
   // Taking care of recurrence rule
-  let recurrenceRule: string | undefined = undefined;
+  let recurrenceRule: string | undefined;
   const icsRole: ParticipationRole = "REQ-PARTICIPANT";
   if (event.recurringEvent?.count) {
     // ics appends "RRULE:" already, so removing it from RRule generated string
@@ -107,7 +107,7 @@ const generateIcsString = ({
         : []),
     ],
     location: location ?? undefined,
-    method: "REQUEST",
+    method: status === "CANCELLED" ? "CANCEL" : "REQUEST",
     status,
     ...(event.hideCalendarEventDetails ? { classification: "PRIVATE" } : {}),
     busyStatus: "BUSY",