fix(search_jobs): prevent job state flip-flopping between 'queued' and 'processing' (#64098)

Fixes SPLF-170

This fixes a bug in our status reporting where we would report a job as
"queued" although it should be reported as "processing". In some cases
this led to the state flip-flopping between "queued" and "processing".

This happens, for example, if a worker has just finished a repo-rev job
(and set it to completed) but hasn't yet dequeued a new job. In this
brief period we reported the entire search job as queued.

Test plan:
- updated unit test
- manual testing

I ran the following search job which previously exhibited this behavior
and verified that now it doesn't.
```
context:global r:^github\.com/sourcegraph/sourcegraph$@*refs/heads/* ghp_.+ patterntype:regexp
```

internal/search/exhaustive/store/exhaustive_search_jobs.go

@@ -264,8 +264,11 @@ const aggStateSubQuery = `
 			CASE
 				WHEN canceled > 0 THEN 'canceled'
 				WHEN processing > 0 THEN 'processing'
-				WHEN queued > 0 THEN 'queued'
+				-- errored jobs will be retried, so we treat them as processing
 				WHEN errored > 0 THEN 'processing'
+				WHEN queued > 0 AND completed is null THEN 'queued'
+				-- this condition is necessary to avoid returning 'queued' when there are no jobs processing at the moment
+				WHEN queued > 0 THEN 'processing'
 				WHEN failed > 0 THEN 'failed'
 				WHEN completed > 0 THEN 'completed'
 			    -- This should never happen

internal/search/exhaustive/store/exhaustive_search_jobs_test.go

@@ -368,7 +368,16 @@ func TestStore_AggregateStatus(t *testing.T) {
 				searchJob: types.JobStateCompleted,
 				repoJobs:  []types.JobState{types.JobStateQueued},
 			},
-			want: types.JobStateQueued,
+			want: types.JobStateProcessing,
+		},
+		{
+			name: "no job is processing, some are completed, some are queued",
+			c: stateCascade{
+				searchJob:   types.JobStateCompleted,
+				repoJobs:    []types.JobState{types.JobStateCompleted},
+				repoRevJobs: []types.JobState{types.JobStateCompleted, types.JobStateQueued},
+			},
+			want: types.JobStateProcessing,
 		},
 		{
 			name: "search job is queued, but no other job has been created yet",
@@ -380,7 +389,7 @@ func TestStore_AggregateStatus(t *testing.T) {
 	}
 
 	for i, tt := range tc {
-		t.Run("", func(t *testing.T) {
+		t.Run(tt.name, func(t *testing.T) {
 			userID, err := createUser(bs, fmt.Sprintf("user_%d", i))
 			require.NoError(t, err)