Ved 1224 update MNS validation method to handle duplicate NHS numbers

[FimranNHS]

Summary

This update enhances the MNS validation logic to correctly handle scenarios where the same NHS number appears multiple times within a batch file. Previously, the batch test failed because duplicate NHS numbers caused the dataframe lookup to overwrite earlier rows, keeping only the last record. Since each event has a unique IMMS_ID, the validator now iterates through all rows for a given NHS number and matches messages using both NHS number and IMMS_ID to identify the correct MNS event.

all functional test passed : https://github.com/NHSDigital/immunisation-fhir-api/actions/runs/24182442830/job/70578590562

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all of the acceptance criteria of the ticket.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• If there were changes that are outside of the regular release processes e.g. account infrastructure to setup, manual setup for external API integrations, secrets to set, then I have checked that the developer has flagged this to the Tech Lead as release steps.
• I have checked that no Personal Identifiable Data (PID) is logged as part of the changes.

[github-actions]

This branch is working on a ticket in the NHS England VED JIRA Project. Here's a handy link to the ticket:

VED-1224

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud