Force date format in context query (#22684) (#22826)

Fixes issue reported in https://discuss.elastic.co/t/view-surrounding-documents-causes-failed-to-parse-date-field-exception/147234

The default date field format is `strict_date_optional_time||epoch_millis`
so we didn't run into this during testing. If the user has custom
mappings for their timestamp field and `epoch_millis` isn't one of the
optional formats the context query will fail since it always sends the
date in millis. This change forces the query to accept the date in millis.

src/core_plugins/kibana/public/context/api/__tests__/predecessors.js

@@ -103,7 +103,7 @@ describe('context app', function () {
           // should have started at the given time
           expect(intervals[0].gte).to.eql(MS_PER_DAY * 3000);
           // should have ended with a half-open interval
-          expect(_.last(intervals)).to.only.have.key('gte');
+          expect(_.last(intervals)).to.only.have.keys('gte', 'format');
           expect(intervals.length).to.be.greaterThan(1);
 
           expect(hits).to.eql(searchSourceStub._stubHits.slice(0, 3));

src/core_plugins/kibana/public/context/api/__tests__/successors.js

@@ -103,7 +103,7 @@ describe('context app', function () {
           // should have started at the given time
           expect(intervals[0].lte).to.eql(MS_PER_DAY * 3000);
           // should have ended with a half-open interval
-          expect(_.last(intervals)).to.only.have.key('lte');
+          expect(_.last(intervals)).to.only.have.keys('lte', 'format');
           expect(intervals.length).to.be.greaterThan(1);
 
           expect(hits).to.eql(searchSourceStub._stubHits.slice(-3));

src/core_plugins/kibana/public/context/api/context.js

@@ -216,6 +216,7 @@ function fetchContextProvider(indexPatterns, Private) {
             filter: {
               range: {
                 [timeField]: {
+                  format: 'epoch_millis',
                   ...startRange,
                   ...endRange,
                 }