test: expand handler tests to 53.4% coverage

internal/service/interest_test.go

@@ -64,3 +64,34 @@ func TestCalculateInterestForDeposit_ExpiredDeposit(t *testing.T) {
 		t.Errorf("expected empty result for expired deposit, got %q", result)
 	}
 }
+
+func TestCalculateInterestForDeposit_WrongDay(t *testing.T) {
+	s := &InterestService{}
+	// Start date with day that is NOT today
+	// We pick a day that will never be today unless very lucky
+	today := time.Now()
+	// Pick a start day that differs from today
+	startDay := today.Day() + 1
+	if startDay > 28 {
+		startDay = 1
+	}
+	startDate := time.Date(today.Year()-1, today.Month(), startDay, 0, 0, 0, 0, time.UTC)
+	
+	deposit := &model.SavingsCategory{
+		Name:         "Test",
+		IsDeposit:    true,
+		InterestRate: 12,
+		DepositStartDate: sql.NullTime{
+			Time:  startDate,
+			Valid: true,
+		},
+	}
+	// Should return empty string (not today's interest day) — no DB call
+	result, err := s.CalculateInterestForDeposit(deposit)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result != "" {
+		t.Errorf("expected empty result for non-interest day, got %q", result)
+	}
+}