fix: gate #[mutants::skip] behind cfg(test) for downstream compat (0.3.3)

The #[mutants::skip] proc macro from the mutants dev-dep was not available
to downstream users, causing compilation failure. Gate it with cfg_attr(test).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ucobs"
-version = "0.3.2"
+version = "0.3.3"
 edition = "2021"
 rust-version = "1.93"
 license = "MIT"