New 'Fable' model-picker entitlement probe with retry/backoff and telemetry

A new 'Fable' probe checks whether the account is entitled to a given model before offering it in the model picker, caching in-flight probes and tracking attempt counts against a cap with retry/backoff. It emits model_picker_fable_probe telemetry events with outcomes accepted, refused, or failed, including a superseded_credential discard case.