Drop the manual web.search_backend / web.backend config-reading block
that duplicated _read_config_key in web_search_registry.py. The function
now delegates directly to get_active_search_provider() (which reads the
same config keys via the registry's canonical resolver) and falls back
to _get_search_backend() only when the registry has no providers loaded.
Also updates the TestXaiWebSearchBackendPreference tests to monkeypatch
the registry instead of load_config_readonly, and adds two new tests for
the legacy fallback path (no provider registered -> _get_search_backend).
f5be9236e0