Get queryable history coverage

Returns the first and last eligible observation dates at which /rates with the same base, quotes and providers returns at least one record. Defaults to the EUR-based daily blended feed. Supplying providers selects their history, including non-daily observations. Null bounds mean no queryable pair. Bounds are request dates, not necessarily returned record dates: a carried quote can predate the bridge required to convert its base. Interior gaps are possible. Bounds do not extend beyond stored observation dates merely because carry-forward can answer later requests. With several quotes, at least one must be available. Only base, quotes and providers are accepted. Use start_date from this endpoint for a headline describing default rate history; provider and currency publication dates are not pair coverage.