Rename ensure_autocommit_off() to ensure_autocommit_on(), closes #705

The ensure_autocommit_off() context manager was misleadingly named: it
sets isolation_level = None on the underlying sqlite3 connection, which
puts the driver INTO autocommit mode (no implicit transactions) - the
opposite of what the name and docstring claimed.

The behavior was always correct for its call sites, which need
autocommit to run PRAGMA statements like journal_mode=wal. This renames
the method to ensure_autocommit_on() with an accurate docstring, updates
the three internal call sites, adds tests, and documents the rename in
the changelog and the 3.x to 4.0 upgrading guide. The old name is
removed outright since 4.0 permits breaking changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_wal.py

@@ -49,6 +49,17 @@ def test_disable_wal_inside_transaction_raises(db_path_tmpdir):
     assert [r["id"] for r in db["test"].rows] == [1]
 
 
+def test_ensure_autocommit_on(db_path_tmpdir):
+    db, path, tmpdir = db_path_tmpdir
+    previous_isolation_level = db.conn.isolation_level
+    assert previous_isolation_level is not None
+    with db.ensure_autocommit_on():
+        # isolation_level of None means driver-level autocommit mode
+        assert db.conn.isolation_level is None
+    # Restored afterwards
+    assert db.conn.isolation_level == previous_isolation_level
+
+
 def test_enable_wal_noop_inside_transaction_is_allowed(db_path_tmpdir):
     # Calling enable_wal() when WAL is already enabled is a no-op,
     # so it is fine inside a transaction