feat: add compression engine test helpers and refactor tests

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/tests/common/test_helpers.rs

@@ -62,3 +62,30 @@ pub fn create_test_item(conn: &Connection) -> i64 {
     };
     db::insert_item(conn, item).expect("Failed to insert item")
 }
+
+/// Create a test file with compression engine testing data
+pub fn create_test_file_with_data(dir: &TempDir, filename: &str, data: &[u8]) -> PathBuf {
+    let file_path = dir.path().join(filename);
+    let mut file = File::create(&file_path).expect("Failed to create test file");
+    file.write_all(data).expect("Failed to write to test file");
+    file_path
+}
+
+/// Test compression and decompression with an engine
+pub fn test_compression_engine(engine: &dyn crate::compression_engine::CompressionEngine, test_data: &[u8]) {
+    let dir = create_temp_dir();
+    let file_path = dir.path().join("test_compression.dat");
+    
+    // Test compression
+    {
+        let mut writer = engine.create(file_path.clone()).expect("Failed to create writer");
+        writer.write_all(test_data).expect("Failed to write data");
+    }
+    
+    // Test decompression
+    let mut reader = engine.open(file_path).expect("Failed to open reader");
+    let mut decompressed = Vec::new();
+    std::io::copy(&mut reader, &mut decompressed).expect("Failed to read data");
+    
+    assert_eq!(test_data, decompressed.as_slice());
+}