Simplify batching query in PatientStatusUpdater

We've seen some evidence that the batching queries generated by
`PatientStatusUpdater#update_programme_statuses!` are (at least sometimes) slow
in production, potentially because of a bunch of sequential scans relating to
JOINs generated by the call to `ActiveRecord::QueryMethods#includes`.

The call to `#includes` is necessary in order to trigger eager loading of
associations to avoid N+1 queries when calling
`Patient::ProgrammeStatus#assign`. However, this is only relevant for the batch
queries which make the model instances available within each batch; not for the
batching queries which just work out which model IDs to include in a given
batch.

Thus we can simplify the batching queries without changing the overall
behaviour by using `ActiveRecord::Batches#in_batches` [1] instead of
`#find_in_batches` and moving the call to `#includes` onto the batch relation.

Note that as per the `ActiveRecord::QueryMethods#includes` docs [2]:

> A separate query is performed for each association, unless a join is required
> by conditions.

This means that the JOINs were only being generated on the batching queries
when the patient scope included a condition, e.g. when associated with an
`ImmunisationImport`. Given the instance of `PatientStatusUpdaterJob` scheduled
overnight [3] uses the default arguments, its batching query is *unscoped* and
thus it may not directly benefit from any performance improvements made by the
changes in this commit.

[1]: https://api.rubyonrails.org/v8.1.3/classes/ActiveRecord/Batches.html#method-i-in_batches
[2]: https://api.rubyonrails.org/v8.1.3/classes/ActiveRecord/QueryMethods.html#method-i-includes
[3]: https://github.com/NHSDigital/manage-vaccinations-in-schools/blob/6895ee514a018aaa507a460e332a32b3d9e8b6a1/config/sidekiq.yml#L70-L73

Co-authored-by: Chris Lowis <chris.lowis@gofreerange.com>
Co-authored-by: Chris Roos <chris.roos@gofreerange.com>

Author: 3 people

app/lib/patient_status_updater.rb

@@ -42,16 +42,18 @@ def update_programme_statuses!
 
     merge_patient_scope(Patient::ProgrammeStatus)
       .where(academic_year: academic_years)
-      .includes(
-        :attendance_record,
-        :consents,
-        :patient,
-        :patient_locations,
-        :triages,
-        :vaccination_records,
-        :parents
-      )
-      .find_in_batches do |batch|
+      .in_batches do |relation|
+        batch =
+          relation.includes(
+            :attendance_record,
+            :consents,
+            :patient,
+            :patient_locations,
+            :triages,
+            :vaccination_records,
+            :parents
+          ).to_a
+
         batch.each(&:assign)
 
         Patient::ProgrammeStatus.import!(