Return error for duplicate assignee IDs in UpdateAssigneesService

What does this MR do and why?

Fixes a silent rejection bug in MergeRequests::UpdateAssigneesService where passing duplicate IDs in assignee_ids caused the update to be silently ignored with no error returned to the caller.

The fix adds an explicit duplicate check before processing: if assignee_ids contains duplicate values, the service now adds a validation error ("Duplicate assignee IDs are not allowed") to the merge request and returns early, making the failure visible to callers (e.g. the API returns a 422 with the error message).

A corresponding RSpec test is added to cover this new error path.

References

Closes #601085 (closed)

Screenshots or screen recordings

N/A — backend-only change.

Before	After

How to set up and validate locally

1. In a Rails console or via the API, send a PUT request to /api/v4/projects/:id/merge_requests/:mr_iid with assignee_ids: [<id>, <id>] (same ID twice).
2. Before this fix: HTTP 200 with no change and no error.
3. After this fix: HTTP 422 with { "message": { "assignees": ["Duplicate assignee IDs are not allowed"] } }.

MR acceptance checklist

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