I get a 500 error when I try to re-invite an already invited user.

## Impact

Users receive a 500 error when attempting to re-invite an already invited user. This prevents them from successfully sending invitations.

## Expected behaviour

The system should return a 409 status code when trying to re-invite a user who has already been invited.

## Actual behaviour

The current behaviour of the system returns a 500 error status when re-inviting an already invited user.

## Steps to reproduce

- Use core v25.06 (10.122.12.221).
- Attempt to re-invite a user who has already been invited.

## Environment

- Core version: v25.06 (10.122.12.221)
- Need to backport v25.06, v24.12

## Workarounds

N/A

## Other information

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JIRA Issue: BA-1902