fix: combine handshake into a single write

Send protocol version and client registration in one write+drain
instead of two, matching the Go client's behavior.

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

Author: antoineleclair

src/dqliteclient/protocol.py

@@ -47,13 +47,9 @@ async def handshake(self, client_id: int = 0) -> int:
 
         Returns the heartbeat timeout from server.
         """
-        # Send protocol version
-        self._writer.write(self._encoder.encode_handshake())
-        await self._writer.drain()
-
-        # Send client registration
+        # Send protocol version + client registration together
         request = ClientRequest(client_id=client_id)
-        self._writer.write(request.encode())
+        self._writer.write(self._encoder.encode_handshake() + request.encode())
         await self._writer.drain()
 
         # Read welcome response