docs: fix README usage example that crashed at runtime

The example encoded a LeaderRequest (type 0) then decoded it in response
mode where type 0 is FailureResponse, causing DecodeError. Replaced with
working encode_message/decode_message example and removed unused import.

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

Author: antoineleclair

README.md

@@ -11,17 +11,14 @@ pip install dqlite-wire
 ## Usage
 
 ```python
-from dqlitewire import MessageEncoder, MessageDecoder
-from dqlitewire.messages import LeaderRequest, ClientRequest
+from dqlitewire import encode_message, decode_message
+from dqlitewire.messages import LeaderRequest
 
 # Encode a message
-encoder = MessageEncoder()
-data = encoder.encode(LeaderRequest())
+data = encode_message(LeaderRequest())
 
 # Decode a message
-decoder = MessageDecoder()
-decoder.feed(data)
-message = decoder.decode()
+message = decode_message(data, is_request=True)
 ```
 
 ## Thread-safety

tests/test_codec.py

@@ -1895,3 +1895,16 @@ def test_partial_feed_chunked(self) -> None:
         assert isinstance(decoded, ResultResponse)
         assert decoded.last_insert_id == 42
         assert decoded.rows_affected == 7
+
+
+class TestReadmeExample:
+    """Verify the README usage example actually works."""
+
+    def test_readme_example_runs_without_error(self) -> None:
+        """The README shows encode_message / decode_message with a LeaderRequest."""
+        from dqlitewire import decode_message, encode_message
+        from dqlitewire.messages import LeaderRequest
+
+        data = encode_message(LeaderRequest())
+        message = decode_message(data, is_request=True)
+        assert isinstance(message, LeaderRequest)