[feat] Add comprehensive unit tests for TaskQueue operations

- Add MockTaskQueue class with dependency injection for isolated testing
- Test core operations: queueing, processing, batch tracking, state management
- Test thread safety: concurrent access, worker lifecycle, exception handling
- Test integration workflows: full task processing with WebSocket updates
- Test edge cases: empty queues, invalid data, cleanup scenarios
- Solve heapq compatibility by wrapping items in priority tuples
- Include pytest configuration and test runner script
- All 15 tests passing with proper async/threading support

Testing covers:
✅ Task queueing with Pydantic validation
✅ Batch history tracking and persistence
✅ Thread-safe concurrent operations
✅ Worker thread lifecycle management
✅ WebSocket message delivery tracking
✅ State snapshots and error conditions

tests/__init__.py

@@ -0,0 +1 @@
+"""Test suite for ComfyUI-Manager"""