feat(slots): add observability to guest-busy-times degradation path

The catch arm in _getGuestBusyTimesForReschedule silently returned [] on
any failure to keep rescheduling unblocked. That is the right runtime
behaviour, but a silent swallow makes upstream regressions (e.g. a Prisma
schema drift in BookingRepository.findByUidIncludeAttendeeEmails) look
like 'no Cal.com guests found' rather than a real fault.

Emit a structured warn through the existing slots/util logger so operators
can detect this without paging on a non-blocking code path. Uses
safeStringify (already imported and used elsewhere in this file) so the
error never breaks the log line.

Author: Bcornish

packages/trpc/server/routers/viewer/slots/util.ts

@@ -887,7 +887,13 @@ export class AvailableSlotsService {
 
       return guestBookings.map((b) => ({ start: b.startTime, end: b.endTime }));
     } catch (error) {
-      // Graceful degradation: never block rescheduling if guest lookup fails
+      // Graceful degradation: never block rescheduling if guest lookup fails.
+      // Log at warn (not error) so operators can detect upstream regressions
+      // without paging on a non-blocking code path.
+      log.warn(
+        "[getGuestBusyTimesForReschedule] degraded to empty result",
+        safeStringify({ rescheduleUid, error })
+      );
       return [];
     }
   }