fix: validate version parameter in MessageDecoder constructor

MessageDecoder now rejects unsupported protocol versions at construction
time for response decoders, matching the validation already present in
MessageEncoder. Request decoders ignore the version parameter since it
comes from the handshake.

Closes #122

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

Author: antoineleclair

src/dqlitewire/codec.py

@@ -175,6 +175,11 @@ def __init__(self, is_request: bool = False, version: int = PROTOCOL_VERSION) ->
                     for pre-1.0 dqlite servers (affects LeaderResponse format).
                     Ignored for request decoders (version comes from handshake).
         """
+        if not is_request and version not in _SUPPORTED_VERSIONS:
+            raise ProtocolError(
+                f"Unsupported protocol version: {version:#x}. "
+                f"Supported: {', '.join(f'{v:#x}' for v in sorted(_SUPPORTED_VERSIONS))}"
+            )
         self._buffer = ReadBuffer()
         self._is_request = is_request
         self._type_map = REQUEST_TYPES if is_request else RESPONSE_TYPES

tests/test_codec.py

@@ -80,6 +80,21 @@ def test_encode_message(self) -> None:
 
 
 class TestMessageDecoder:
+    def test_decoder_rejects_invalid_version(self) -> None:
+        """122: response decoder must reject invalid version at construction."""
+        with pytest.raises(ProtocolError, match="Unsupported protocol version"):
+            MessageDecoder(version=0xDEADBEEF)
+
+    def test_decoder_accepts_legacy_version(self) -> None:
+        """122: PROTOCOL_VERSION_LEGACY must be accepted."""
+        decoder = MessageDecoder(version=PROTOCOL_VERSION_LEGACY)
+        assert decoder.version == PROTOCOL_VERSION_LEGACY
+
+    def test_decoder_request_ignores_version(self) -> None:
+        """122: request decoders ignore version param (comes from handshake)."""
+        decoder = MessageDecoder(is_request=True, version=0xDEADBEEF)
+        assert decoder.version is None  # not set until handshake
+
     def test_decode_handshake(self) -> None:
         decoder = MessageDecoder(is_request=True)
         decoder.feed(PROTOCOL_VERSION.to_bytes(8, "little"))