Spark Connect: server only handles one phase per start (ZinggCommandPlugin stops the shared session)

[Chandanydv1234]

Branch: spark-connect

Over Spark Connect, only the first phase works. Any phase after it fails with
"Cannot call methods on a stopped SparkContext".

Cause: ZinggCommandPlugin calls client.stop() -> session.stop(), which shuts
down the server's shared SparkSession. Fine for the CLI (one Spark per run),
wrong for a shared Connect server.

Fix: don't call client.stop() in the plugin.