refactor(example-images): minimize async lock contention by moving I/O outside critical sections

- Extract progress file loading to async methods to run in executor
- Refactor start_download to reduce lock time by pre-loading data before entering lock
- Improve check_pending_models efficiency with single-pass model collection and async loading
- Add type hints to get_status method
- Add tests for download task callback execution and error handling

tests/utils/test_example_images_download_manager_unit.py

@@ -203,3 +203,150 @@ async def test_pause_or_resume_without_running_download(
 
     with pytest.raises(download_module.DownloadNotRunningError):
         await manager.stop_download(object())
+
+
+async def test_download_task_callback_executes_on_completion(
+    monkeypatch: pytest.MonkeyPatch, tmp_path
+) -> None:
+    """Test that _handle_download_task_done callback is executed when download completes."""
+    settings_manager = get_settings_manager()
+    settings_manager.settings["example_images_path"] = str(tmp_path)
+    settings_manager.settings["libraries"] = {"default": {}}
+    settings_manager.settings["active_library"] = "default"
+
+    ws_manager = RecordingWebSocketManager()
+    manager = download_module.DownloadManager(ws_manager=ws_manager)
+
+    callback_executed = asyncio.Event()
+    original_callback = manager._handle_download_task_done
+
+    def tracking_callback(task, output_dir):
+        original_callback(task, output_dir)
+        callback_executed.set()
+
+    monkeypatch.setattr(
+        manager, "_handle_download_task_done", tracking_callback
+    )
+
+    async def fake_download(self, *_args):
+        # Simulate successful completion
+        async with self._state_lock:
+            self._progress["status"] = "completed"
+            self._is_downloading = False
+            self._download_task = None
+
+    monkeypatch.setattr(
+        download_module.DownloadManager,
+        "_download_all_example_images",
+        fake_download,
+    )
+
+    result = await manager.start_download({"model_types": ["lora"], "delay": 0})
+    assert result["success"] is True
+
+    # Wait for callback to execute
+    await asyncio.wait_for(callback_executed.wait(), timeout=1)
+    assert manager._progress["status"] == "completed"
+
+
+async def test_download_task_callback_handles_errors(
+    monkeypatch: pytest.MonkeyPatch, tmp_path
+) -> None:
+    """Test that _handle_download_task_done properly handles task errors and saves progress."""
+    settings_manager = get_settings_manager()
+    settings_manager.settings["example_images_path"] = str(tmp_path)
+    settings_manager.settings["libraries"] = {"default": {}}
+    settings_manager.settings["active_library"] = "default"
+
+    ws_manager = RecordingWebSocketManager()
+    manager = download_module.DownloadManager(ws_manager=ws_manager)
+
+    callback_executed = asyncio.Event()
+    progress_saved = False
+    original_save_progress = manager._save_progress
+
+    def tracking_save_progress(output_dir):
+        nonlocal progress_saved
+        progress_saved = True
+        return original_save_progress(output_dir)
+
+    monkeypatch.setattr(manager, "_save_progress", tracking_save_progress)
+
+    original_callback = manager._handle_download_task_done
+
+    def tracking_callback(task, output_dir):
+        original_callback(task, output_dir)
+        callback_executed.set()
+
+    monkeypatch.setattr(
+        manager, "_handle_download_task_done", tracking_callback
+    )
+
+    async def fake_download_with_error(self, *_args):
+        raise RuntimeError("Simulated download error")
+
+    monkeypatch.setattr(
+        download_module.DownloadManager,
+        "_download_all_example_images",
+        fake_download_with_error,
+    )
+
+    result = await manager.start_download({"model_types": ["lora"], "delay": 0})
+    assert result["success"] is True
+
+    # Wait for callback to execute (it should handle the error)
+    await asyncio.wait_for(callback_executed.wait(), timeout=1)
+    # Progress should be saved even on error
+    assert progress_saved is True
+
+
+async def test_get_status_returns_correct_state(
+    monkeypatch: pytest.MonkeyPatch, tmp_path
+) -> None:
+    """Test that get_status returns the correct download state."""
+    settings_manager = get_settings_manager()
+    settings_manager.settings["example_images_path"] = str(tmp_path)
+    settings_manager.settings["libraries"] = {"default": {}}
+    settings_manager.settings["active_library"] = "default"
+
+    ws_manager = RecordingWebSocketManager()
+    manager = download_module.DownloadManager(ws_manager=ws_manager)
+
+    # Test idle state
+    status = await manager.get_status(object())
+    assert status["success"] is True
+    assert status["is_downloading"] is False
+    assert status["status"]["status"] == "idle"
+
+    started = asyncio.Event()
+    release = asyncio.Event()
+
+    async def fake_download(self, *_args):
+        started.set()
+        await release.wait()
+        async with self._state_lock:
+            self._is_downloading = False
+            self._download_task = None
+
+    monkeypatch.setattr(
+        download_module.DownloadManager,
+        "_download_all_example_images",
+        fake_download,
+    )
+
+    # Start download
+    result = await manager.start_download({"model_types": ["lora"], "delay": 0})
+    assert result["success"] is True
+
+    await asyncio.wait_for(started.wait(), timeout=1)
+
+    # Test running state
+    status = await manager.get_status(object())
+    assert status["success"] is True
+    assert status["is_downloading"] is True
+    assert status["status"]["status"] == "running"
+
+    # Cleanup
+    release.set()
+    if manager._download_task:
+        await asyncio.wait_for(manager._download_task, timeout=1)