Replace single-client model (m_client/m_readBuffer/m_initialized)
with a ClientState vector. Each client gets its own read buffer and
initialized flag. Responses route to m_currentSender (set during
request processing); notifications broadcast to all initialized
clients.
Re-entrancy guard in onReadyRead: re-resolve ClientState after each
processLine() call since sendJson flush can re-enter the event loop
and trigger onDisconnected, removing the client mid-iteration.
Tests: 378-line test_mcp exercising connect, initialize, tools/list,
disconnect one client, notification broadcast, and serial requests
against a MockMcpServer with the same multi-client architecture.
