feat: add tests and integrate dots.ocr model

G.2.5 - Tests:
- Add pytest test suite with fixtures
- test_preprocessing.py - PDF/image loading, normalization, validation
- test_postprocessing.py - chunks, QA pairs, markdown generation
- test_inference.py - dummy parser and inference functions
- test_api.py - API endpoint tests
- Add pytest.ini configuration

G.1.3 - dots.ocr Integration:
- Update model_loader.py with real model loading code
  - Support for AutoModelForVision2Seq and AutoProcessor
  - Device handling (CUDA/CPU/MPS) with fallback
  - Error handling with dummy fallback option
- Update inference.py with real model inference
  - Process images through model
  - Generate and decode outputs
  - Parse model output to blocks
- Add model_output_parser.py
  - Parse JSON or plain text model output
  - Convert to structured blocks
  - Layout detection support (placeholder)

Dependencies:
- Add pytest, pytest-asyncio, httpx for testing

services/parser-service/pytest.ini

@@ -0,0 +1,14 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+addopts = 
+    -v
+    --tb=short
+    --strict-markers
+markers =
+    unit: Unit tests
+    integration: Integration tests
+    slow: Slow running tests
+