Return full OUTPUT row for composite primary key inserts

[Copilot]

SQL Server inserts with composite primary keys were not assigning all database-generated key parts back to the created Active Record instance. In practice, only part of the returned key data was applied, leaving generated components (e.g., uniqueidentifier) unset on the in-memory record.

• Adapter behavior change

  • Override SQL Server returning_column_values to return the full first returned row, not a single scalar.
  • This aligns insert-return handling with multi-column OUTPUT INSERTED... payloads used for composite keys.

• Regression coverage

  • Add a targeted test for a table with composite PK (id IDENTITY, guid DEFAULT NEWID()).
  • Assert that create! assigns both generated PK components (id and guid) to the created instance.

# SQL Server adapter: preserve all returned columns for insert RETURNING/OUTPUT
def returning_column_values(result)
  result.rows.first
end