chore: Add missing doc strings for spark client functions

[chahatsagarmain]

What this PR does / why we need it:

This PR updates the SparkClient public API methods (list_sessions, get_session, delete_session, and get_session_logs) to include complete Google-style docstrings.
Previously, these methods were missing detailed parameter, return, and exception documentations. These changes ensure the docstrings now comprehensively detail Args, Returns, and Raises sections, aligning with the codebase's documentation standards.

Which issue(s) this PR fixes

Fixes #462

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[Copilot]

Pull request overview

This PR improves the public SparkClient API documentation by expanding several method docstrings to match the repository’s Google-style docstring standards (per Issue #462).

Changes:

• Add Returns documentation for list_sessions.
• Add Args/Returns/Raises documentation for get_session, delete_session, and get_session_logs.

[Copilot]

list_sessions() docstring documents Returns but omits Raises, even though the backend can raise TimeoutError and RuntimeError (see RuntimeBackend.list_sessions). Add a Raises: section to keep the public API docs complete and consistent with the documented backend behavior.

Suggested change

	List of SparkConnectInfo objects for all sessions in the namespace.
	List of SparkConnectInfo objects for all sessions in the namespace.
	Raises:
	RuntimeError: If the sessions cannot be retrieved.
	TimeoutError: If the request to the Kubernetes API times out.