Default Req adapter for LLM.HTTPClient.
Wraps the Req library to implement the
LLM.HTTPClient behaviour. This is the production HTTP client used
unless overridden via config :llm, :http_client.
In tests, swap this out with a Mox mock:
Mox.defmock(LLM.HTTPClient.Mock, for: LLM.HTTPClient)
config :llm, :http_client, LLM.HTTPClient.Mock