Extract Rubydex native adapter helpers

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Problems

• Several native extension adapters repeated the same handle-to-graph unpacking sequence.
• Rust-owned C string conversion repeated the same NULL handling, Ruby string creation, and free_c_string ownership pattern.

Solutions

• Add rdxi_graph_from_handle to centralize handle and graph typed-data unpacking while preserving the existing Ruby graph object ownership.
• Add rdxi_owned_c_string_to_ruby for Rust-owned strings and use it only at existing free_c_string callsites.