fix: verify response type in finalize() for protocol consistency

Every other protocol method checked its expected response type, but
finalize() silently accepted any non-failure response. Now raises
ProtocolError if the response is not EmptyResponse.

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

Author: antoineleclair

src/dqliteclient/protocol.py

@@ -9,6 +9,7 @@
 from dqlitewire.messages import (
     ClientRequest,
     DbResponse,
+    EmptyResponse,
     ExecSqlRequest,
     FailureResponse,
     FinalizeRequest,
@@ -138,6 +139,9 @@ async def finalize(self, db_id: int, stmt_id: int) -> None:
         if isinstance(response, FailureResponse):
             raise OperationalError(response.code, response.message)
 
+        if not isinstance(response, EmptyResponse):
+            raise ProtocolError(f"Expected EmptyResponse, got {type(response).__name__}")
+
     async def exec_sql(
         self, db_id: int, sql: str, params: Sequence[Any] | None = None
     ) -> tuple[int, int]:

tests/test_protocol.py

@@ -85,6 +85,20 @@ async def test_open_database_failure(
         assert exc_info.value.code == 1
         assert "cannot open" in exc_info.value.message
 
+    async def test_finalize_wrong_response_type(
+        self,
+        protocol: DqliteProtocol,
+        mock_reader: AsyncMock,
+    ) -> None:
+        """finalize() should reject non-EmptyResponse (catches protocol desync)."""
+        from dqlitewire.messages import DbResponse
+
+        # Server sends wrong response type
+        mock_reader.read.return_value = DbResponse(db_id=99).encode()
+
+        with pytest.raises(ProtocolError, match="Expected EmptyResponse"):
+            await protocol.finalize(1, 1)
+
     async def test_exec_sql(
         self,
         protocol: DqliteProtocol,