Re-enable clippy::empty_enums after never_type is stable

[coderabbitai]

Summary

Re-enable and address the clippy::empty_enums lint after Rust stabilizes the required never_type support.

Rationale

Pull request #2499 disables clippy::empty_enums across the Rust workspace because Rust v1.98 does not support the lint's suggested never_type replacement as a stable feature.

Affected area

• .cargo/config.toml
• Rust crates in the workspace that contain empty enums

Required changes

1. Confirm that the Rust toolchain used by this repository supports never_type as stable.
2. Remove the workspace-level exception for clippy::empty_enums.
3. Replace affected empty enums with the supported never_type representation where appropriate.
4. Run the repository's Rust lint checks.

Acceptance criteria

• The supported Rust toolchain has stable never_type support.
• clippy::empty_enums is no longer disabled in .cargo/config.toml.
• Clippy passes without violations of clippy::empty_enums.

Backlinks

• Pull request: chore(lint): Allow clippy::empty_enums across the Rust workspace. #2499
• Request comment: chore(lint): Allow clippy::empty_enums across the Rust workspace. #2499 (comment)
• Requested by: @LinZhihao-723