chore(spark): add SparkClient API delegation coverage

[lntutor]

Summary

• add unit coverage for SparkClient.connect() against existing Spark Connect servers, including URL validation and token propagation
• cover create-mode forwarding of session configuration to the Kubernetes backend
• cover public session-management delegation, including delete_session() returning None and get_session_logs() forwarding follow by keyword

Part of #609. This is intentionally limited to the Spark API layer; it does not overlap with #610, #614, or #621.

Validation

• uv run pytest -q kubeflow/spark/api/spark_client_test.py — 15 passed
• uv run ruff check kubeflow/spark/api/spark_client_test.py
• uv run ruff format --check kubeflow/spark/api/spark_client_test.py
• make verify

I also ran make test-python; it entered the unrelated Trainer container-backend test section and did not complete locally, so I stopped that single runner rather than leave it consuming resources. The focused Spark suite is clean and CI will run the repository gate.

[google-oss-prow]

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[github-actions]

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[lntutor]

Current head f5d9337248adb54764fb36d89653696db76d6b9f is mergeable. The earlier failing Check PR Title run has been superseded by later successful runs on this head, so there is no current title-check failure to fix in the branch. The remaining gate appears to be the Kubeflow/Prow approval path: tide is still pending, and the bot notes that after review with lgtm, approval from an approver such as @electronic-waste is needed. If the change looks good, a maintainer/approver can move it forward from here.