Move the generic async coroutine out of the host crate into litellm-coroutine,
with its requirements documented in the crate's AGENTS.md. RouteMachine becomes
CallMachine on top of it, host protocol ops move to host/src/protocol.rs, and
the messages, OCR, host-python, python-bridge and legacy callbacks crates adopt
the new types. Adds error definition rules to litellm-rust/AGENTS.md.
Co-authored-by: Yujong Lee <yujong@berri.ai>
