Agnes/lg 13013 billing report multiple year helper

[samathad2023]

🎫 Ticket

Link to the relevant ticket: LG-13013

🛠 Summary of changes

Added new helper for unique user auth counts by partner.

📜 Testing Plan

Provide a checklist of steps to confirm the changes.

• [ ] Unit Test to cover new helper function
• [ ] Negative Test scenarios : empty values and partial month
• [ ] Positive Test scenarios: ial 1 &2 with billable true/false