chore: add unused_async clippy lint, deny unreachable_pub

[noot]

📝 Summary

• add unused_async clippy lint
• deny unreachable_pub; this previously was not being caught by CI, as it's a rust lint, and the CI was running make lint and only denying on clippy warnings

💡 Motivation and Context

code quality

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[Copilot]

Pull Request Overview

This PR enhances code quality by adding stricter linting rules: the unused_async clippy lint to catch unnecessary async functions, and upgrading unreachable_pub from warn to deny to enforce proper visibility modifiers.

• Added unused_async clippy lint to warn about unnecessary async functions
• Changed unreachable_pub lint level from warn to deny
• Removed unnecessary async keyword from generate_genesis function

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
Cargo.toml	Added unused_async clippy lint and changed unreachable_pub to deny level
crates/op-rbuilder/src/tests/framework/apis.rs	Removed unnecessary async modifier from generate_genesis function

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