Split out from WT-18230. This ticket covers the API check and fixes tests that use invalid schema epochs.

Checkpoint below a published drop

BFG-3485199, BFG-3499971, and BFG-3508343 checkpointed a table after it had been dropped and recreated, using an epoch below the drop.
A checkpoint below a published drop is legal if no recreate is queued. It becomes an API violation when a later CREATE for the same table is part of that checkpoint. Checkpoints covering the recreate remain legal.
Add this check to __wt_disagg_shared_metadata_queue_process. Update the schema-abort test so it does not recreate a table until a checkpoint covers its published drop.

Invalid Python reproducer

The reproducer switches from epoch-less operation to schema epochs midway through the test, so the recreated table does not receive the AWAITS_PUBLISH flag.
Set the stable schema epoch before the first operation, publish every schema operation, and checkpoint at the drop epoch while the recreate remains deferred. Cover the invalid drop-and-recreate sequence separately with a panic test.