test(models/inference): add comprehensive unit tests for inference engine

Add test_inference.py with full coverage of models.inference module:

TestGenerationConfig:
- Test default and custom configuration values
- Test to_dict serialization

TestInferenceResult:
- Test default and populated result objects
- Test dictionary conversion

TestExtractSqlFromOutput:
- Test extraction from plain SQL, markdown blocks
- Test XML tags, SQL prefix, WITH/INSERT/UPDATE/DELETE
- Test whitespace handling and no-SQL fallback

TestCalculateConfidence:
- Test empty scores handling
- Test confidence calculation with token probabilities
- Test confidence bounds (0-1)

TestInferenceEngine:
- Test initialization and properties
- Test generate with SQL extraction
- Test token limit and inference exceptions
- Test batch generation with error handling

TestGetInferenceEngine:
- Test singleton pattern

37 tests covering SQL extraction, confidence scoring, and batch processing

Part of Phase 1.3: HuggingFace Model Integration (Issue #3)

Author: Sakeeb91