Enable rustapi-testing as dev-dependency in rustapi-core

Uncommented and activated the rustapi-testing crate in rustapi-core's dev-dependencies to support testing. Updated Cargo.lock to reflect the new dependency.

Author: Tuntii

Cargo.lock

@@ -79,7 +79,7 @@ rcgen = { workspace = true, optional = true }
 [dev-dependencies]
 tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
 proptest = "1.4"
-# rustapi-testing = { workspace = true }
+rustapi-testing = { workspace = true }
 reqwest = { version = "0.12", features = ["json", "stream"] }
 async-stream = "0.3"
 [features]