Extend `Session.post` to allow batched message posting

libbmq provides `bmqa::MessageEventBuilder` to batch multiple messages into a single message event. However, `pybmq_session` does not expose any API for batching messages, creating a new `bmqa::MessageEventBuilder` for each posted message. For parity with libbmq, we should expose the ability to batch messages somehow.

Start by reading Session.post in pybmq_session and how it currently creates bmqa::MessageEventBuilder instances. Compare this with libbmq's batching behavior and determine an API shape for posting multiple messages in one event. Done means pybmq_session exposes batching with parity to libbmq.

Written by the indexing model from the issue text.