add tests for llms

semantic_router/llms/base.py

tests/unit/llms/test_llm_base.py

tests/unit/llms/test_llm_cohere.py

tests/unit/llms/test_llm_openai.py

tests/unit/llms/test_llm_openrouter.py

tests/unit/test_route.py

tests/unit/test_schema.py