[Workflows] Better handling for existing user email collisions

Workflow collect-data form returns server error when updating a user email to an existing value.

When a customer reaches a workflow Collect Data step and submits a form containing the user email field, the submission can fail with an internal server error if the entered email is already used by another user in the same organization.

The backend correctly detects the uniqueness conflict, but currently surfaces it as a 500 instead of a user-friendly validation error.

This makes the workflow appear broken even though the root cause is a duplicate unique identifier value.

Repro:

1. Create an end-user with email A

2. Connect to Webchat as a user with email B

3. Once the workflow asks for email - provide email A

4. Email field is displaying server error on submit - unable to continue

The system should treat it as an identity-resolution case rather than a generic validation failure - i.e. either merge user with duplicate, switch initially submitted email to the newer one or switch user to a lead perhaps