fix(usage): display cumulative spend on zero event days (#87243)

For reserved budget categories, we always display the usage chart in
spend mode. When you switch type to cumulative it'd only show the
cumulative spend on days that had spend. This PR fixes that so that on
days without spend, we just use the previous day's spend. For periodic
usage, behavior remains the same as before (only show spend for that
day, in which case days without spend will have 0).

# before 

![image](https://github.com/user-attachments/assets/65bbdad1-c2c9-46c4-9538-954f8a2452c1)

# after

![image](https://github.com/user-attachments/assets/e89494ee-f2fd-4ec8-9087-12c721b849ce)

Author: isabellaenriquez

static/gsApp/views/subscriptionPage/reservedUsageChart.tsx

@@ -361,6 +361,8 @@ export function mapReservedBudgetStatsToChart({
     return chartData;
   }
 
+  let previousReservedForDate = 0;
+  let previousOnDemandForDate = 0;
   Object.entries(statsByDateAndCategory).forEach(([date, statsByCategory]) => {
     let reservedForDate = 0;
     let onDemandForDate = 0;
@@ -408,13 +410,26 @@ export function mapReservedBudgetStatsToChart({
         }
       });
     });
+    // if cumulative and there was no new spend on this date, use the previous date's spend
+    if (
+      reservedForDate === 0 &&
+      isCumulative &&
+      moment(date).isSameOrBefore(moment().toDate())
+    ) {
+      reservedForDate = previousReservedForDate;
+    }
+    if (onDemandForDate === 0 && isCumulative) {
+      onDemandForDate = previousOnDemandForDate;
+    }
     const dateKey = getDateFromMoment(moment(date));
     chartData.reserved!.push({
       value: [dateKey, reservedForDate],
     });
+    previousReservedForDate = reservedForDate;
     chartData.onDemand!.push({
       value: [dateKey, onDemandForDate],
     });
+    previousOnDemandForDate = onDemandForDate;
   });
 
   return chartData;