fix(routing): preserve explicit backend selectors when static_route is configured

BackendModelResolver no longer applies static_route override when the
request model contains an explicit backend selector (backend:model).
This ensures requests like 'ollama:glm-5.1:cloud' are not incorrectly
rewritten to the static_route backend.

Changes:
- Added request_has_explicit_backend_selector check before applying override
- Static route only applies to backend-less model selectors
- Added debug logging for both override and skip paths
- Added regression tests at unit and API levels

Author: Mateusz