Move the shared test helpers out of litellm-inference's test-support
feature into a publish = false litellm-inference-testing crate used only
as a dev dependency by the format crates.
Also drop the dead src/constants.rs (OPENAI_DEFAULT_API_BASE had no
users) and declare the litellm-http/litellm-llms test-support features
on the crates that actually use them instead of relying on feature
unification through litellm-inference's dev-dependencies.
Co-authored-by: Yujong Lee <yujong@berri.ai>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
