[Investigations] - Add _source back to security solution flyout (#180477)

Resolves https://github.com/elastic/security-team/issues/8806

<img width="1421" alt="image"
src="c58abb22-cd76-4531-b764-de99036923a1">

x-pack/plugins/timelines/server/search_strategy/timeline/factory/events/details/index.ts

@@ -39,7 +39,8 @@ export const timelineEventsDetails: TimelineFactory<TimelineEventsQueries.detail
     response: IEsSearchResponse<EventHit>
   ): Promise<TimelineEventsDetailsStrategyResponse> => {
     const { indexName, eventId, runtimeMappings = {} } = options;
-    const { fields, ...hitsData } = response.rawResponse.hits.hits[0] ?? {};
+    // _source is removed here as it's only needed in the rawEventData below
+    const { fields, _source, ...hitsData } = response.rawResponse.hits.hits[0] ?? {};
 
     const inspect = {
       dsl: [

x-pack/plugins/timelines/server/search_strategy/timeline/factory/events/details/query.events_details.dsl.test.ts

@@ -22,7 +22,7 @@ describe('buildTimelineDetailsQuery', () => {
       Object {
         "allow_no_indices": true,
         "body": Object {
-          "_source": false,
+          "_source": true,
           "fields": Array [
             Object {
               "field": "*",

x-pack/plugins/timelines/server/search_strategy/timeline/factory/events/details/query.events_details.dsl.ts

@@ -61,7 +61,7 @@ export const buildTimelineDetailsQuery = ({
       // Remove and instead pass index_pattern.id once issue resolved: https://github.com/elastic/kibana/issues/111762
       runtime_mappings: runtimeMappings,
       stored_fields: ['*'],
-      _source: false,
+      _source: true,
     },
     size: 1,
   };