Background: The test key provider extension in ext/test/key_provider/key_provider.c currently only implements the pull-based get_key model. For the push-based set_key API introduced in WT-17535 and WT-17536 to be exercised by the Python suite and Catch2 tests, the extension needs a real set_key implementation that stores pushed key data and its associated timestamp.

Currently there is no test harness capable of exercising the push model end-to-end.

Acceptance Criteria:

• key_provider.c implements set_key, storing pushed key data and timestamp
• Extension registers with version=1 so WiredTiger wires up set_key and never calls get_key
• Existing key state tracking (KEY_STATE_CURRENT, KEY_STATE_PENDING, KEY_STATE_READ) is updated or replaced to reflect the push model lifecycle
• Extension can be used as-is by the existing Python suite and Catch2 tests