Improve AI impact analytics dashboard trend indicator tooltips

What does this MR do and why?

Adds a new Incomparable data message when the change % cannot be calculated due to insufficient data, or a divide by zero error. Refactor the change calculation into a separate util method to simplify the test suite

MR acceptance checklist

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Screenshots or screen recordings

Unable to calculate when data is missing

Change from 0 to 0

Change from 4 to 0 (metric in the last column is an incomplete data set, so we use the 2nd last column instead)

Change from 0 to 2

Change from '-' to 2

How to set up and validate locally

1. Configure your GDK to use ClickHouse

# enable clickhouse
Feature.enable(:clickhouse_data_collection)
Feature.enable(:event_sync_worker_for_click_house)

# enable the worker to sync code suggestions events to clickhouse
Feature.enable(:code_suggestion_events_in_click_house)

2. Ensure that you are using GitLab Ultimate
3. Create a new group
4. Navigate to the Analytics dashboard page for the new group (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards)
5. Select the AI impact analytics dashboard
6. Verify that the Change % column tooltip is appropriate for the data provided

---

I recommend mocking the data using this patch, as it is difficult to assign specific values to columns of the table

diff --git a/ee/app/assets/javascripts/analytics/dashboards/ai_impact/components/metric_table.vue b/ee/app/assets/javascripts/analytics/dashboards/ai_impact/components/metric_table.vue
index e70165069dab..5c2744bc28ca 100644
--- a/ee/app/assets/javascripts/analytics/dashboards/ai_impact/components/metric_table.vue
+++ b/ee/app/assets/javascripts/analytics/dashboards/ai_impact/components/metric_table.vue
@@ -157,7 +157,25 @@ export default {
 
     async fetchTableMetrics({ metrics, queryFn }) {
       try {
-        const data = await fetchMetricsForTimePeriods(DASHBOARD_TIME_PERIODS, queryFn);
+        let data = await fetchMetricsForTimePeriods(DASHBOARD_TIME_PERIODS, queryFn);
+
+        const metricId = 'vulnerability_critical';
+        const metricValues = [0, 5, 6, 1, 2, 3];
+        data = data.map(
+          (timePeriod, index) => {
+            if (metricId in timePeriod) {
+              return {
+                ...timePeriod,
+                vulnerability_critical: {
+                  identifier: metricId,
+                  value: metricValues[index],
+                },
+              };
+            }
+            return timePeriod;
+          },
+        );
+
         this.tableData = mergeTableData(this.tableData, generateTableRows(data));
       } catch (error) {
         throw metrics;

Related to #462713 (closed)