Support sqlite3.connect(autocommit=False) connections too

Database() now accepts all three Python transaction handling modes and
behaves identically in each. For autocommit=False connections - where
the driver holds an implicit transaction open at all times - the
library tracks transaction ownership itself:

- A new _explicit_transaction flag distinguishes transactions opened
  with begin()/atomic() from the driver's implicit one, exposed as a
  new db.in_transaction property (conn.in_transaction is always True
  in this mode)
- begin() claims the driver's implicit transaction instead of
  executing BEGIN, which the driver would reject; BEGIN/COMMIT/
  ROLLBACK passed to db.execute() are routed through begin()/commit()/
  rollback()
- Writes outside a user transaction commit the implicit transaction
  immediately, preserving the library's auto-commit contract
- Row-returning statements outside a transaction fetch eagerly and
  commit, so the implicit read transaction does not hold a shared
  lock that blocks writes from other connections
- PRAGMA and VACUUM run in temporary driver autocommit mode
  (ensure_autocommit_on() now flips conn.autocommit), since PRAGMAs
  are silently ignored and VACUUM refused inside the implicit
  transaction

A new pytest --sqlite-autocommit-false option runs the entire suite
in this mode, wired into CI alongside --sqlite-autocommit. Tests that
asserted on conn.in_transaction or wrote through db.conn without
committing now use the mode-aware library API instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PFkrS2nuP8mo1jmT594VCd

tests/test_cli.py

@@ -2200,7 +2200,7 @@ def test_search_quote(tmpdir):
 def test_indexes(tmpdir):
     db_path = str(tmpdir / "test.db")
     db = Database(db_path)
-    db.conn.executescript("""
+    db.executescript("""
         create table Gosh (c1 text, c2 text, c3 text);
         create index Gosh_idx on Gosh(c2, c3 desc);
     """)
@@ -2294,7 +2294,7 @@ def test_triggers(tmpdir, extra_args, expected):
         pk="id",
     )
     db["counter"].insert({"count": 1})
-    db.conn.execute(textwrap.dedent("""
+    db.execute(textwrap.dedent("""
         CREATE TRIGGER blah AFTER INSERT ON articles
         BEGIN
             UPDATE counter SET count = count + 1;