fix/test: increase LLM test coverage and fix validation bugs

[SeanClay10]

Summary

• Fixed a Pydantic v2 bug where Optional[Annotated[str, constr(...)]] silently ignored string constraints (pattern, min_length, max_length) on species_name, study_location, and study_date in PredatorDietMetrics — moved constraints into Field() where they are actually enforced
• Added tests/test_models.py (38 tests) covering field construction, sample_size auto-reconciliation, fraction_feeding computation, field-level validation, and JSON round-trip serialization
• Added tests/test_llm_text.py (47 tests) covering page splitting, section priority scoring, paragraph keyword scoring, sentence truncation, budget-bounded section extraction, and document loading
• Consolidated duplicate tests in test_pdf_classifier.py, test_pdf_extraction.py, and test_train_model.py
• Fixed test_save_to_file_permission_error which always failed on Windows due to a hardcoded /dev/null path