Fix SAWarning in lambda_stmt test

[flipbit03]

Summary

• Replace subqueryload with selectinload in test_lambda_stmt_with_subqueryload test
• Rename test to test_lambda_stmt_with_selectinload_multiple_children to avoid duplicate names

Why

SQLAlchemy emits a warning when using subqueryload with lambda_stmt:

SAWarning: subqueryloader for "SDParent.children" must invoke lambda callable... Consider using selectinload() for more effective full-lambda caching

This is because subqueryload doesn't cache efficiently with lambda statements.

Test plan

• All tests pass
• Lint passes (make lint)
• Type check passes (make typecheck)
• No SAWarning when running with -W error::sqlalchemy.exc.SAWarning