fix: detect plugin entry in new Codex navigation

[luSkyl]

Summary

• stop relying on the legacy button.h-token-nav-row.w-full navigation class to find the plugin entry
• detect plugin entry buttons via the plugin icon path and generic button, [role=button] controls
• add a regression assertion so the injected script no longer contains the stale navigation class selector

Testing

• Static regression checks against the generated injection script
• Not run: cargo test locally because the Windows machine used for the fix does not have Rust/Cargo installed