Copy subscription buffer backend options

[yuanknv]

Summary

• deep-copy acceptable Buffer backend strings into subscription-owned options
• initialize copied option storage before assignment
• preserve the original error when option copying fails

Dependency

Independent correctness fix discovered while exercising Rust native Buffer subscriptions.

Test plan

• Build and run the rcl test suite
• Exercise subscription initialization and finalization with CUDA backend options
• Verify the previous invalid free is absent