fix(pytest): add connection handshake to test_subscriptions_blocking

Without a handshake, the blocking pub-sub test relied solely on _wait_until_subscribed(), which exits after receiving the first subscription notification. With multiple subscriptions, later subscription events may not have propagated to the XPUB socket before publishing starts, causing ZMQ to silently drop those messages and the 1-second receiver timeout to fire.

Added a Connected-event ping-pong loop (mirroring _ensure_connected in test_subscriptions_async) so all subscriptions are guaranteed to be active before any test event is sent.

Fixes #371 (closed)

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

Closes #371 (closed)