Background: The existing key provider tests (test_key_provider_disagg01/02.py and Catch2 unit tests) were written against the pull-based get_key model. With the push-based set_key API now implemented (WT-17536) and the test extension updated (WT-17538), the tests need to be updated to exercise the new model and cover scenarios specific to the push design.

Currently the test suite has no coverage of push-model behavior.

Acceptance Criteria:

• Create new push-based python tests to validate new push method.
• Catch2 unit tests updated to reflect push model lifecycle
• New test coverage for: correct key selected when multiple set_key calls happen between checkpoints, timestamp validation errors (decreasing timestamp, timestamp <= stable timestamp), load_key receives correct timestamp on checkpoint open, concurrent set_key and checkpoint