Return structured conflict details from UserMergeToRef

Problem

UserMergeToRef currently has no structured error type of its own, so a caller cannot distinguish a merge conflict from any other failure short of pattern-matching the plain-text error message. Sibling RPCs such as UserMergeBranch already expose a dedicated error message with a merge_conflict variant, but UserMergeToRef has no equivalent to attach as error detail.

Solution

Add the UserMergeToRefError message, mirroring the shape used by UserMergeBranchError: a oneof with a single merge_conflict variant wrapping MergeConflictError. This commit only adds the proto definition and its generated Go code; nothing produces this detail yet, so the RPC's behaviour is unchanged. A following commit wires UserMergeToRef to actually populate it.

Closes #7330 (closed)