Fix 3-month lookback period key mismatch in reports (#5781)

Story card:
[SIMPLEBACK-87](https://rtsl.atlassian.net/browse/SIMPLEBACK-87)

### Because
Reporting logic uses a 3-month internal lookback buffer to calculate
correct patient denominators while displaying only an 18-month UI
window.

### This address
Ensures the first visible month in reports correctly includes patients
registered 3 months earlier, preventing incorrect denominator counts.

### Test instructions

Run repository specs and verify the 3-month lookback reports return the
correct denominator values.

Author: Gyan-Gupta-Rtsl

spec/models/reports/repository_spec.rb

@@ -938,11 +938,11 @@
   context "period extension for 3-month lookback (18 month UI window + 3 month buffer)" do
     let(:facility) { create(:facility, facility_group: facility_group_1) }
 
-    let(:end_period) { Period.month("February 2026") }
+    let(:end_period) { Period.current }
     let(:start_period) { end_period.advance(months: -(Reports::MAX_MONTHS_OF_DATA - 1)) }
     let(:original_range) { (start_period..end_period) }
 
-    let(:repo) { Reports::Repository.new(facility.region, periods: original_range) }
+    let(:single_period) { end_period }
 
     before do
       allow(Reports::PatientState).to receive(:get_refresh_months).and_return(
@@ -951,16 +951,46 @@
           end_period.to_date
         )
       )
+
+      diabetes_patients = create_list(:patient, 3, :diabetes,
+        recorded_at: start_period.advance(months: -Reports::REGISTRATION_BUFFER_IN_MONTHS).to_date,
+        assigned_facility: facility,
+        registration_user: user)
+
+      diabetes_patients.each do |p|
+        create(:blood_sugar, :with_encounter,
+          patient: p,
+          facility: facility,
+          recorded_at: start_period.to_date,
+          user: user)
+      end
+
+      hypertension_patients = create_list(:patient, 3, :hypertension,
+        recorded_at: start_period.advance(months: -Reports::REGISTRATION_BUFFER_IN_MONTHS).to_date,
+        assigned_facility: facility,
+        registration_user: user)
+
+      hypertension_patients.each do |p|
+        create(:bp_with_encounter,
+          patient: p,
+          facility: facility,
+          recorded_at: start_period.to_date,
+          user: user)
+      end
+
+      refresh_views
     end
 
     it "keeps original UI period window as 18 months" do
+      repo = Reports::Repository.new(facility.region, periods: original_range)
+
       expect(repo.periods).to eq(original_range)
-      expect(repo.periods.count).to eq(18)
+      expect(repo.periods.count).to eq(Reports::MAX_MONTHS_OF_DATA)
     end
 
     [:adjusted_diabetes_patients_with_ltfu, :adjusted_patients_with_ltfu].each do |method|
       it "does not expose buffer months for #{method}" do
-        refresh_views
+        repo = Reports::Repository.new(facility.region, periods: original_range)
         result = repo.send(method)[facility.region.slug]
 
         expect(result.keys.min).to eq(start_period)
@@ -972,90 +1002,37 @@
     end
 
     it "ensures first visible month has access to 3-month back denominator data" do
-      patients = create_list(:patient, 3, :diabetes,
-        recorded_at: start_period.advance(months: -3).to_date,
-        assigned_facility: facility,
-        registration_user: user)
+      repo = Reports::Repository.new(facility.region, periods: original_range)
 
-      patients.each do |patient|
-        create(:blood_sugar, :with_encounter, patient: patient, facility: facility, recorded_at: start_period.to_date, user: user)
-      end
-
-      refresh_views
       result = repo.adjusted_diabetes_patients_with_ltfu[facility.region.slug]
       expect(result[start_period]).to eq(3)
     end
 
     it "works correctly for single period input with 3-month lookback data" do
-      single_period = Period.month("February 2026")
-
-      patients = create_list(:patient, 2, :diabetes,
-        recorded_at: single_period.advance(months: -3).to_date,
-        assigned_facility: facility,
-        registration_user: user)
-
-      patients.each do |patient|
-        create(:blood_sugar, :with_encounter,
-          patient: patient,
-          facility: facility,
-          recorded_at: single_period.to_date,
-          user: user)
-      end
-
       repo = Reports::Repository.new(facility.region, periods: single_period)
 
-      refresh_views
       result = repo.adjusted_diabetes_patients_with_ltfu[facility.region.slug]
+
       expect(repo.periods).to eq(single_period..single_period)
       expect(result.keys).to eq([single_period])
-      expect(result[single_period]).to eq(2)
+      expect(result[single_period]).to eq(3)
     end
 
     it "ensures first visible month has access to 3-month back denominator data for hypertension" do
-      patients = create_list(:patient, 3, :hypertension,
-        recorded_at: start_period.advance(months: -3).to_date,
-        assigned_facility: facility,
-        registration_user: user)
-
-      patients.each do |patient|
-        create(:bp_with_encounter, patient: patient, facility: facility, recorded_at: start_period.to_date, user: user)
-      end
+      repo = Reports::Repository.new(facility.region, periods: original_range)
 
-      refresh_views
       result = repo.adjusted_patients_with_ltfu[facility.region.slug]
       expect(result[start_period]).to eq(3)
     end
 
     it "works correctly for single period input with 3-month lookback data for hypertension" do
-      single_period = Period.month("February 2026")
-
-      allow(Reports::PatientState).to receive(:get_refresh_months).and_return(
-        ReportingHelpers.get_refresh_months_between_dates(
-          single_period.advance(months: -Reports::REGISTRATION_BUFFER_IN_MONTHS).to_date,
-          single_period.to_date
-        )
-      )
-
-      patients = create_list(:patient, 2, :hypertension,
-        recorded_at: single_period.advance(months: -3).to_date,
-        assigned_facility: facility,
-        registration_user: user)
-
-      patients.each do |patient|
-        create(:bp_with_encounter,
-          patient: patient,
-          facility: facility,
-          recorded_at: single_period.to_date,
-          user: user)
-      end
-
       repo = Reports::Repository.new(facility.region, periods: single_period)
 
-      refresh_views
       result = repo.adjusted_patients_with_ltfu[facility.region.slug]
+
       expect(repo.periods).to eq(single_period..single_period)
       expect(result.keys).to eq([single_period])
-      expect(result[single_period]).to eq(2)
+      expect(result[single_period]).to eq(3)
     end
   end
 end