test: add 54 unit tests for core evaluation components

[Pranjal0410]

test: add comprehensive unit test suite for core evaluation components

Problem

The project currently has a single integration test (test_integration_real.py) that requires OmniParser model weights and an OpenAI API key to run. This means:

• Tests cannot run in CI without expensive external dependencies
• Core logic bugs (scoring, serialization, data models) go untested
• Contributors cannot verify their changes locally without model weights (~2GB+)

Solution

Added tests/test_unit.py with 42 unit tests across 13 test classes covering all core components that don't require external services:

Test Class	Tests	What It Covers
TestNielsenHeuristics	5	All 10 heuristic definitions, criteria IDs, severity weights
TestSeverityLevel	3	Enum values and string conversion
TestUIElement	9	Creation, properties, computed width/height, serialization, from_dict with both OmniParser and legacy formats, missing field defaults
TestUIElementDetectionResult	2	Full and empty result serialization
TestInferHeadingLevel	4	Heading level inference from bbox height
TestCalculateHeightVariance	4	Variance calculation for consistency checks
TestHeuristicViolation	4	Violation serialization across all severity levels
TestHeuristicScore	5	Score percentage calculation, violation inclusion, LLM explanation handling
TestHeuristicEvaluationResult	3	Full result serialization, ISO timestamp, metadata defaults
TestHeuristicEngineScoring	8	calculate_score() — deductions per severity, accumulation, floor at 0, unknown heuristic handling
TestExceptions	6	Exception hierarchy and field handling
TestHeuristicEngineInit	1	Engine starts uninitialized

Key Design Decisions

• Zero external dependencies: No OmniParser weights, no OpenAI API key, no Firebase needed
• Tests the scoring math directly: Verifies that H1.2 major deducts 6 points, critical deducts 10, etc. against the actual weights in constants.py
• Covers both OmniParser and legacy from_dict formats: Ensures backward compatibility
• Follows existing project patterns: Uses pytest as already used in test_integration_real.py

Running

pip install pytest
pytest tests/test_unit.py -v