Sessions gain a control-only connection mode

What

• Sessions and their control/event channel gained a controlOnly mode, threaded through from session-open callers down to the low-level session-open call (defaulting to unset/false).
• SessionsV2Client appends control_only=1 to the session events stream URL when configured with controlOnly, letting a connection subscribe to control messages only, without the full event stream.
• When a session is opened via attach, the emitted "opened" payload conditionally includes controlOnly: true.

Why

This lets a client connect to a session just to receive control messages, without also receiving the full event stream, which is more efficient when only control signaling is needed.