Update test docstrings to use the public LEADER_ERROR_CODES name

The previous cycle dropped the underscore-prefixed aliases
(_LEADER_CHANGE_CODES in the SA dialect, _LEADER_ERROR_CODES in the
client) in favor of the wire layer's public LEADER_ERROR_CODES name.
Two test files retained the old aliases in their module docstrings,
referencing names that no longer exist anywhere in source. Anyone
grepping for the old names hit these comments without finding the
constant.

Replace the literal _LEADER_CHANGE_CODES and _LEADER_ERROR_CODES in
the docstrings with LEADER_ERROR_CODES. Pure text edit; no test or
runtime change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

tests/test_connection_protocol_error_propagation.py

@@ -4,7 +4,7 @@
 Lines covered (pre-pragma):
 
 - connection.py:408 — ``connect()`` re-raises ``OperationalError``
-  whose code is NOT in ``_LEADER_ERROR_CODES`` (leader-change
+  whose code is NOT in ``LEADER_ERROR_CODES`` (leader-change
   errors translate to ``DqliteConnectionError`` for the pool's
   reconnect-elsewhere flow; non-leader errors propagate as-is).
 - connection.py:531 — ``_abort_protocol`` no-protocol early

tests/test_server_failure_mid_stream.py

@@ -10,7 +10,7 @@
 The dbapi layer then maps ``client.ProtocolError`` to
 ``InterfaceError``, breaking ``sqlalchemy-dqlite``'s ``is_disconnect``
 check which tests ``isinstance(e, OperationalError) and e.code in
-_LEADER_CHANGE_CODES``. Leadership-loss during a streaming SELECT would
+LEADER_ERROR_CODES``. Leadership-loss during a streaming SELECT would
 not trigger SA pool invalidation.
 
 Catching ``ServerFailure`` before the generic ``_WireProtocolError``