fix: reject ClusterRequest format=0 to prevent ServersResponse V0 corruption

ServersResponse.decode_body unconditionally reads a uint64 role field
after each node's address. If a V0-format response (no role field) were
received, the decoder would silently misinterpret subsequent node IDs as
role values, cascading data corruption. ClusterRequest now rejects
format=0 at construction time since ServersResponse only supports V1.

Closes #120

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

Author: antoineleclair

src/dqlitewire/messages/requests.py

@@ -507,6 +507,12 @@ class ClusterRequest(Message):
 
     def __post_init__(self) -> None:
         _check_uint64("format", self.format)
+        if self.format == 0:
+            raise ValueError(
+                "ClusterRequest format=0 (V0) is not supported. "
+                "ServersResponse only decodes V1 format (with node role fields). "
+                "Use format=1."
+            )
 
     def encode_body(self) -> bytes:
         return encode_uint64(self.format)

tests/test_messages_requests.py

@@ -1,5 +1,7 @@
 """Tests for request message encoding/decoding."""
 
+import pytest
+
 from dqlitewire.constants import HEADER_SIZE, RequestType
 from dqlitewire.messages.base import Header
 from dqlitewire.messages.requests import (
@@ -402,6 +404,11 @@ def test_default_format_is_v1(self) -> None:
         msg = ClusterRequest()
         assert msg.format == 1
 
+    def test_format_v0_rejected(self) -> None:
+        """120: V0 cluster format not supported by ServersResponse decoder."""
+        with pytest.raises(ValueError, match="format=0.*not supported"):
+            ClusterRequest(format=0)
+
 
 class TestTransferRequest:
     def test_roundtrip(self) -> None: