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/player/events.rs

@@ -172,10 +172,6 @@ mod tests {
         pub fn events(&self) -> Vec<PlayerStatusEvent> {
             self.events.lock_safe().clone()
         }
-
-        pub fn clear(&self) {
-            self.events.lock_safe().clear();
-        }
     }
 
     impl PlayerEventEmitter for TestEventEmitter {