Simplify SparkConnectState to align with Spark Operator state model

Summary

Review and simplify the SDK SparkConnectState model to better align with the Spark Operator's SparkConnectState enum.

Background

During the review of PR #521, it was noted that:

• RUNNING is not an operator state and may be redundant with READY.
• PROVISIONING is currently unused by the SparkConnect controller.
• The operator exposes a NEW ("") state that is not currently represented in the SDK.

Proposed work

• Review the SDK SparkConnectState model against the current Spark Operator CRD.
• Determine whether RUNNING should be removed or consolidated with READY.
• Evaluate whether PROVISIONING should remain in the SDK.
• Add support for any missing operator states where appropriate.
• Update unit tests and documentation as needed.

Reference

• PR discussion: https://github.com/kubeflow/sdk/pull/521#discussion_r3524198219
• Spark Operator SparkConnectState: https://github.com/kubeflow/spark-operator/blob/master/api/v1alpha1/sparkconnect_types.go#L170-L180