Add a track_degraded parameter to IntervalPool.get_intervals so service calls
(track_degraded=False) never flip the coordinator's sensor-health flags or use
the covers-current-interval cache fallback; authentication errors still
propagate. All price-fetching services now go through
async_fetch_service_intervals and return a structured response with a
top-level success flag: success=false with reason="price_data_unavailable" on
a real API outage, and success=true (possibly empty) otherwise.
Impact: Action calls (get_price, find_cheapest_*, plan_charging) always return
the expected shape, never impair sensors during a Tibber outage, and let
automations distinguish an outage from "no data yet".
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