Downgrade expert mode queries to blocksql

[morgando]

There is a bug when running expert mode under snapshot isolation. Because these queries open cursors without first going through get_prepared_stmt or handle_sql_begin, the snapshot state never gets initialized — which means expert-mode queries end up running with an undefined snapshot.

This PR makes a few changes:

• Downgrades expert-mode snapshot queries to blocksql. This shouldn’t result in unexpected client-facing behavior, since expert-mode queries don’t return result sets and the client can’t actually execute writes in expert mode.
• Adds a test that runs expert mode under snapshot isolation.
• Adds assertions to ensure that any snapshot-transaction opening cursors has its snapshot state initialized — which would have caught the undefined-snapshot issue surfaced by the new test.

[roborivers]

Coding style check: Success ✓.
Smoke testing: Success ✓.
Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
analyze
consumer_non_atomic_default_consumer_generated
insert_lots_ssl_generated
insert_lots
incoh_remsql_rte_connect_generated
incoh_remsql