fix: use correct SQLite extended error codes for leader detection

The previous values {0, 4097} came from Go client internals, not the
wire protocol. The C dqlite server sends SQLite extended error codes:
- SQLITE_IOERR_NOT_LEADER = 10250 (SQLITE_IOERR | (40 << 8))
- SQLITE_IOERR_LEADERSHIP_LOST = 10506 (SQLITE_IOERR | (41 << 8))

Code 0 actually means "empty statement", not "not leader", so the
old constant would invalidate connections on empty SQL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/connection.py

@@ -8,8 +8,10 @@
 from dqliteclient.exceptions import DqliteConnectionError, OperationalError, ProtocolError
 from dqliteclient.protocol import DqliteProtocol
 
-# dqlite error codes that indicate a leader change
-_LEADER_ERROR_CODES = {0, 4097}  # ErrNotLeader, ErrLeadershipLost
+# dqlite error codes that indicate a leader change (SQLite extended error codes)
+# SQLITE_IOERR_NOT_LEADER = SQLITE_IOERR | (40 << 8) = 10250
+# SQLITE_IOERR_LEADERSHIP_LOST = SQLITE_IOERR | (41 << 8) = 10506
+_LEADER_ERROR_CODES = {10250, 10506}
 
 
 def _parse_address(address: str) -> tuple[str, int]:

tests/test_connection.py

@@ -428,8 +428,9 @@ async def test_not_leader_error_invalidates_connection(self) -> None:
 
         assert conn.is_connected
 
-        # Server responds with "not leader" error (code 0)
-        not_leader = FailureResponse(code=0, message="not leader").encode()
+        # Server responds with "not leader" error
+        # SQLITE_IOERR_NOT_LEADER = SQLITE_IOERR | (40 << 8) = 10250
+        not_leader = FailureResponse(code=10250, message="not leader").encode()
         mock_reader.read.side_effect = [not_leader]
 
         from dqliteclient.exceptions import OperationalError