Implement epoch-less schema operations (legacy) in the schema-disagg-abort test.

Epoch-less schema operations:

• Stable epoch timestamp never set; WT runs in legacy mode.
• No publish for schema ops.

Note: epoch-less schema operations can be tested in a single-node mode only. Multi-node needs schema epochs for proper functioning.

Part of this ticket is to assess whether this coverage goes into the existing schema_disagg_abort test (single node only), or warrants a separate, smaller and simpler test.