include_unconfigured was hardcoded True in _handle_model_options, so external
API consumers had no way to request the explicit-only view. Forward the
include_unconfigured query param (default true, back-compatible) to
build_model_options_payload. With _append_unconfigured_rows now honoring
model_catalog.excluded_providers, this endpoint no longer resurrects excluded
providers on any view (#68816).
Also lifts model_options decoding (_request_reasoning_config /
_request_service_tier) into gateway/platforms/api_server_request_options.py to
offset api_server.py's frozen line cap (moved code keeps its cap; the facade
re-exports both names).
82f2dcb286