fix: resolve all Rust test compile warnings without suppression

Remove unused imports and a dead test helper, and strengthen tests that
held unused bindings/fields so the values are actually exercised:
- drop unused imports (HttpConfig, MediaType, std::io::Write)
- remove unused TestEventEmitter::clear helper
- replace meaningless size_of asserts with real empty-manager checks
- assert on MockTrack.name in sorting tests

src-tauri/src/download/mod.rs

@@ -120,7 +120,6 @@ pub struct DownloadTask {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::player::MediaType;
 
     #[test]
     fn test_download_manager_set_max_concurrent() {