Fix assigned endpoint reservation conditions

[wang-tong0]

Summary

• Add :null_type to endpoint reservation conditions only when an attribute_type(..., :null_type) check is present.
• Add a regression test for replacing a fully populated champion/predeploy assignment with a challenger reservation.

Root cause

PR572 always supplied :null_type in ExpressionAttributeValues. When every assignment identity field and the assignment token were populated, the condition expression did not reference that value, so DynamoDB rejected the update with a ValidationException.

Validation

• pytest -q -p no:cacheprovider tests/test_scheduler_provider_dispatch.py tests/test_scheduler_ssh.py tests/test_scheduler_flow.py tests/test_dao_adapters.py
• 168 tests passed.
• Production hotfix validation confirmed UID197 successfully acquired the active endpoint reservation without the previous :null_type error.