docs: document encode_message contract change

Note in the protocol API docs that both contracted _ProtocolHandler
methods receive the connection's negotiated ProtocolFeatures, spelling
out the calling conventions: decode_message receives it positionally,
encode_message as the required protocol_features keyword argument, so
overrides must keep that parameter name. Add a CHANGELOG entry with
upgrade guidance for custom protocol handlers (accept protocol_features,
prefer **kwargs for future-proofing, forward it when delegating to
send_body).

Author: nikagra

CHANGELOG.rst

@@ -1,3 +1,21 @@
+Unreleased
+==========
+
+Others
+------
+* Message serialization now receives the connection's negotiated ``ProtocolFeatures``:
+  ``Connection.send_msg`` passes ``protocol_features`` to the encoder, and
+  ``_ProtocolHandler.encode_message`` forwards it to each message's ``send_body``.
+  This changes the contracted signature of ``encode_message`` (and of ``send_body``).
+  Custom protocol handlers that override ``encode_message`` must accept a required
+  ``protocol_features`` keyword argument (adding ``**kwargs`` is recommended for
+  future-proofing), and custom encoders that delegate to ``msg.send_body`` should
+  forward it. There is deliberately no compatibility fallback: protocol extensions
+  are negotiated per connection at STARTUP, so an encoder unaware of
+  ``protocol_features`` could silently omit fields a negotiated extension requires.
+  This release emits no new bytes on the wire; the parameter is groundwork for
+  upcoming protocol extensions (``SCYLLA_USE_METADATA_ID``, ``TABLETS_ROUTING_V2``).
+
 3.29.11
 =======
 Jun 15, 2026

docs/api/cassandra/protocol.rst

@@ -27,6 +27,11 @@ See :meth:`.Session.execute`, :meth:`.Session.execute_async`, :attr:`.ResponseFu
 
     .. automethod:: decode_message
 
+.. note::
+    Both contracted methods receive the ``ProtocolFeatures`` negotiated on the connection
+    carrying the message: ``decode_message`` positionally, ``encode_message`` as the required
+    ``protocol_features`` keyword argument (overrides must keep that parameter name).
+
 .. _faster_deser:
 
 Faster Deserialization