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/thumbnail/cache.rs

@@ -344,7 +344,6 @@ mod tests {
     use super::*;
     use crate::storage::db_service::RusqliteService;
     use rusqlite::Connection;
-    use std::io::Write;
     use std::sync::{Arc, Mutex};
     use tempfile::TempDir;