Migrate 18 provider test files from tests/test_litellm/llms to tests/unit/llms.
194 kept tests move as-is after mutation testing; 1 test deleted
(test_completion_datarobot_with_environment_variables, env-gated no-assert);
the fixture-only fal_ai cost calculator file is removed.
Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
