test(demo): add unit and integration coverage for pipeline

Introduce focused unit, integration, and NATS-path tests for demo modules, and align assertions with final schema and temporal contracts (window int, seq=30, fill-level ratio). This commit isolates validation logic from runtime changes and provides reproducible QA for pipeline behavior and failure modes.

tests/demo/conftest.py

@@ -0,0 +1,23 @@
+"""Test fixtures for demo package."""
+
+import numpy as np
+import pytest
+import torch
+
+
+@pytest.fixture
+def mock_frame_tensor():
+    """Return a mock video frame tensor (C, H, W)."""
+    return torch.randn(3, 224, 224)
+
+
+@pytest.fixture
+def mock_frame_array():
+    """Return a mock video frame as numpy array (H, W, C)."""
+    return np.random.randn(224, 224, 3).astype(np.float32)
+
+
+@pytest.fixture
+def mock_video_sequence():
+    """Return a mock video sequence tensor (T, C, H, W)."""
+    return torch.randn(16, 3, 224, 224)