tidy(versioning): extract read_row_outside_flush helper

baseline.py:_insert_baseline_row and changes.py:_read_pre_state both
issued the same "read a single row through ``session.connection()``
inside ``with session.no_autoflush:``" pattern. Same five-line block,
same intent ("read the pre-flush state without triggering the in-flight
edit's flush").

Promoted to ``superset.versioning.utils.read_row_outside_flush(session,
table, entity_id)``. Companion to ``single_flush_scope`` — they sit
next to each other in utils.py and frame the two directions of the
"don't autoflush mid-listener" pattern.

Returns ``dict[str, Any]`` (or ``None``) so callers can't accidentally
hold a cursor-bound ``RowMapping`` past the listener boundary. Both
call sites get shorter by ~5 lines.

Also picks up Decimal stringification in the changes.py docstring
update (was listed in the W4 commit but the docstring still said
"(datetime, UUID, bytes)" — now matches the implementation).

Behaviour unchanged. 96 unit tests pass.