This change makes the API surface between MainTx and App smaller.
Currently, App reconstructs a database transaction by unpacking the isolation
level, transaction mode, DbHandler, and transaction runner returned by MainTx.
That exposes MainTx internals at the call site even though MainTx already owns
query setup, execution, decoding, and rollback behavior.
The goal is to keep transaction assembly in MainTx while App remains responsible
for pool execution, database error mapping, and response orchestration. DbTx now
carries the assembled SQL session, and App passes that session directly to the
connection pool.
This splits off the transaction concerns into a MainTx module
It's just moving some functions inside the Query module to the MainTx module.
Some types names were also changed so they don't have the "Query" prefix, to avoid confusion.
