Use cuda::stream_ref for central stream helpers

[vyasr]

Description

This Wave 2 follow-up to #23691 migrates the central stream helpers from rmm::cuda_stream_view to cuda::stream_ref, including get_default_stream(), default_stream_value, fork_streams, and join_streams.

It also updates the direct Java, pylibcudf, and libcudf_streaming call sites whose method calls depend on those helper return types. rmm::cuda_stream ownership remains unchanged.

This should land after the Wave 1 downstream stream_ref PRs.

Replaces the fork PR vyasr#9.

Contributes to #23636

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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