* refactor(router): move litellm/router.py to litellm/router_python_core.py
A pure rename with no content change, so git and review tools track the router body as a move.
The next commit turns it into PythonCore and adds the Router facade back at litellm/router.py
* refactor(router): extract PythonCore behind RouterCore and pass the router its config
The routing logic moves unchanged from litellm/router.py into PythonCore in
litellm/router_python_core.py. Router becomes a facade that builds a RouterConfig, picks a
RouterCore with make_core and delegates every member to it. The core no longer reads or writes
litellm.* directly: process settings come from RouterDynamicConfig, and inference calls, sleeps,
callback lists and the global cache go through PythonHost. A CI check bans litellm.* in the router
modules, and the proxy's construction sites build the config outside the router
Settings are static: the constructor arguments and changeset values are copied when the router is
built or the change applied, and a write to a setting attribute becomes apply(ChangeSet(assign=...)).
A Router subclass's core class also subclasses the facade, so an override's super(Sub, self) works,
and an instance-level patch of a core method is kept on the facade so nested patches unwind.
tests/unit/test_router/core tests the core in isolation: ScriptedHost serves every host operation
(scripted inference answers, recorded sleeps, settings, callbacks, cache), and make_core runs each
test once per core in CORE_BUILDERS
