[Mocks] Add Mock Server Implementation

**Is your feature request related to a problem? Please describe.**
I would like to unit test this library, but there is no mock server implementation. Testing requires me to start a standalone pulsar server and emit to it.

**Describe the solution you'd like**
I would like to be able to write unit tests without starting up a pulsar server. I want to ensure the server receives messages in the right order

Start by reviewing the repository's existing unit tests and how they currently connect to a standalone Pulsar server. Define the mock server's required interface and message-order checks, then verify that client tests can exercise message delivery without starting a Pulsar server.

Written by the indexing model from the issue text.