docs: warn that HeartbeatRequest is not handled by the dqlite server

The C dqlite server omits HEARTBEAT from its gateway dispatcher, and
the Go client's heartbeat code is commented out. Sending HeartbeatRequest
results in a FailureResponse. Updated the docstring to warn about this
and document the expected response type (ServersResponse) if heartbeat
is ever enabled.

Closes #141

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

Author: antoineleclair

src/dqlitewire/messages/requests.py

@@ -76,6 +76,14 @@ class HeartbeatRequest(Message):
     """Send heartbeat to server.
 
     Body: uint64 timestamp
+
+    Note: The C dqlite server does not currently handle this request type
+    (HEARTBEAT is omitted from the gateway dispatcher). The Go client's
+    heartbeat code is also commented out. Sending this request will result
+    in a FailureResponse from the server.
+
+    If heartbeat is ever enabled, the expected response is a
+    ServersResponse (type 3) containing the cluster node list.
     """
 
     MSG_TYPE: ClassVar[int] = RequestType.HEARTBEAT