### Why are the changes needed?
Since https://github.com/apache/kyuubi/pull/3618
Kyuubi server could retry opening the engine when encountering a special error.
1937dd93f9/kyuubi-server/src/main/scala/org/apache/kyuubi/session/KyuubiSessionImpl.scala (L177-L212)
The `_client` might be reset and closed.
So, we shall set `_client` after open engine session successfully, as the `client` method is a public method.
### How was this patch tested?
Existing UT.
### Was this patch authored or co-authored using generative AI tooling?
No.
Closes#7011 from turboFei/client_ready.
Closes#7011
3ad57ee91 [Wang, Fei] fix npe
b956394fa [Wang, Fei] close internal engine client
523b48a4d [Wang, Fei] internal client
5baeedec1 [Wang, Fei] Revert "method"
84c808cfb [Wang, Fei] method
8efaa52f6 [Wang, Fei] check engine launched
Authored-by: Wang, Fei <fwang12@ebay.com>
Signed-off-by: Wang, Fei <fwang12@ebay.com>
d6f07a6b64