Use a self-join for UPDATE with outer joins

[aidanharan]

Port of rails/rails#54722 for MSSQL

Fix following tests from CI run https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/actions/runs/14035778078/job/39293342462

• UpdateAllTest#test_update_all_with_left_joins_composite_primary_key_can_reference_joined_table:
• UpdateAllTest#test_update_all_with_self_left_joins_can_reference_joined_table:
• UpdateAllTest#test_update_all_with_string_joins_can_reference_joined_table:
• UpdateAllTest#test_update_all_with_left_outer_joins_can_reference_joined_table:

[Copilot]

Pull Request Overview

A port of Rails changes to support UPDATE statements with self-joins for outer joins on MSSQL.

• Update the UPDATE syntax to include an alias for the target table.
• Refactor join injection logic and the prepare_update_statement method to add a self-join.

Comments suppressed due to low confidence (2)

lib/arel/visitors/sqlserver.rb:43

• [nitpick] The alias '__active_record_update_alias' is introduced here, but later in prepare_update_statement the alias is not applied. Consider clarifying the alias usage for consistency between the UPDATE syntax and the self-join approach.

#   UPDATE t1 AS __active_record_update_alias

lib/arel/visitors/sqlserver.rb:71

• Prepending the target table to the join sources may affect join ordering. Validate that this reordering does not interfere with other parts of the query processing that rely on a specific join sequence.

stmt.relation.right = [stmt.relation.left, *stmt.relation.right]