tsk-34: Fetch next ready scheduled message (#38)

Closes #34

Reviewed-on: phoenix/textsender-api#38
Co-authored-by: phoenix <kundeng00@pm.me>
Co-committed-by: phoenix <kundeng00@pm.me>

internal/store/mock/scheduled_message_store.go

@@ -47,6 +47,30 @@ func (m *MockScheduledMessageStore) Get(ctx context.Context, id uuid.UUID) (*sch
 	}
 }
 
+func (m *MockScheduledMessageStore) FetchNextScheduledMessage(ctx context.Context) (*scheduling.ScheduledMessage, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.Error != nil {
+		return nil, m.Error
+	}
+
+	var schMsg *scheduling.ScheduledMessage
+	for _, msg := range m.ScheduledMessages {
+		if msg.Status == scheduling.Ready {
+			msg.Status = scheduling.Processing
+			schMsg = msg
+			break
+		}
+	}
+
+	if schMsg == nil {
+		return nil, fmt.Errorf("No scheduled message is ready")
+	} else {
+		return schMsg, nil
+	}
+}
+
 func (m *MockScheduledMessageStore) GetWithUserId(ctx context.Context, userId uuid.UUID) ([]*scheduling.ScheduledMessage, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()