Add AutoAssignReviewersWorker and reviewer-preserving assign logic (3b/3)

Summary

Introduces the Sidekiq worker that performs automatic reviewer assignment and refactors ReviewerAssignment::AssignService so the worker can invoke it cleanly.

Worker (MergeRequests::AutoAssignReviewersWorker):

• idempotent!, data_consistency :sticky, urgency :low
• feature_category :code_review_workflow
• defer_on_database_health_signal :gitlab_main
• Resolves the automation_bot for the project's organization and passes it into the service as current_user

AssignService refactor:

• Resolves the strategy from StrategyFactory rather than taking it as a required arg, so the worker can call .new(merge_request:, current_user:)
• Takes current_user: in initialize like every other BaseProjectService subclass — no more nil-then-reassign inside execute
• Preserves existing reviewers when assigning (append rather than overwrite)
• Duo Code Review compatible: a solo Duo bot reviewer is treated as "no human reviewers" so code owners are still added alongside

No callers invoke the worker yet; the trigger points arrive in Part 3c.

Part 3b of 3 for CODEOWNERS automatic reviewer assignment. This MR can merge independently of 3a (the service still reads reviewer_auto_assignment_enabled?, which exists on master).

Related

• Parent issue: #589700
• Previously combined in: !224178 (closed) (now split)
• Part 1 (DB): !224175 (merged)
• Part 2 (Service): !224176 (merged)
• Part 3a (Settings toggle): !232619 (merged)
• Part 3c (Trigger wiring): !232621 (merged)

Test plan

• bundle exec rspec ee/spec/workers/merge_requests/auto_assign_reviewers_worker_spec.rb
• bundle exec rspec ee/spec/services/merge_requests/reviewer_assignment/assign_service_spec.rb
• Enqueue worker manually for an MR with existing human reviewer: reviewers remain unchanged (skip path)
• Enqueue worker for an MR with solo Duo bot reviewer: code owners added alongside Duo bot
• Enqueue worker for an MR with no reviewers: code owners assigned, author is automation_bot