Handle claim conflict when creating placeholder users

Sam Wordrequested to merge
600255-handle-claim-conflict-placeholder-users into master

What does this MR do and why?

In a multi-cell cluster, saving a placeholder user during an import may trigger a cluster-wide claim lease that raises Cells::TransactionRecord::AlreadyClaimedError (gRPC ALREADY_EXISTS) when the username, email, or route is already claimed by another cell.

Gitlab::Import::SourceUserMapper#create_source_user_mapping did not rescue this error, so it propagated and crashed the import. This change treats it like the existing PG::UniqueViolation and ActiveRecord::RecordNotUnique collisions by raising DuplicatedUserError so that callers will treat AlreadyClaimedErrors like other non-unique placeholder errors.

How to set up and validate locally

Ensure RSpec tests pass and importer behavior is unaffected

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Resolves #600255 (closed)

Edited by Sam Word

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