Fix Rust 1.93 unsafe attribute and missing return value errors

[Copilot]

Rust 1.93 requires unsafe attributes to be explicitly wrapped in unsafe(...). Additionally, the dump() method's jemallocator branch was missing a return value.

Changes

• lib.rs line 7: Wrap export_name attribute in unsafe(...)

  - #[export_name = "malloc_conf"]
  + #[unsafe(export_name = "malloc_conf")]

• controller.rs line 289: Add explicit Ok(()) return in jemallocator branch

  tokio::fs::write(&file_path, svg).await
      .wrap_err_with(|| format!("Failed to write flamegraph to {}", file_path))?;
  + 
  + Ok(())

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