test: add tempfile and serde_json dev dependencies for unit tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bbelderbos

exercise_downloader/Cargo.lock

@@ -10,3 +10,7 @@ serde = { version = "1.0.227", features = ["derive"] }
 [[bin]]
 name = "pybites-rust-download"
 path = "src/main.rs"
+
+[dev-dependencies]
+serde_json = "1.0.149"
+tempfile = "3.25.0"