List a bookable product's available slots

Returns the computed open slots for a PUBLISHED bookable product over a date range (`?from=&to=`, both required, date-only), scoped to the `:siteId` in the URL and served behind a required storefront publishable key. Optional `offeringId` selects a live formula's effective duration and buffers while the offering-selection flag is on; an invalid id returns 422 `OFFERING_NOT_FOUND`, and omitting it always keeps the normal product/resource grid. An offering whose minimum party size exceeds the storefront party maximum is treated as not selectable: its id resolves to that same 422 `OFFERING_NOT_FOUND` (a buyer could never book it). With the offering flag off, the id is ignored so this shipped route remains available. When the `reservations_resource_pools` flag is on and the product has several interchangeable calendars, availability is the UNION across the pool: a slot is `available` while ANY resource has room, and `seatsRemaining` is the pool's total headroom (ADR-0038 decision F). With the flag off (or a single-calendar product) it is the single calendar's availability, byte-for-byte as before. Each slot carries its instants, an `available` flag (capacity + notice/advance bounds folded in), and `seatsRemaining`; no internal capacity split, tenant, resource, or booking field is surfaced. The notice/advance bounds are the product's EFFECTIVE booking window: a service carrying a per-service override is bounded by that override rather than by its calendar's window, unless the `reservations_per_product_window` kill-switch is forced off. A missing / draft / archived / cross-tenant product, a non-`bookable` product, and a product with no configured calendar all return an identical 404 (no existence oracle). An inverted range is 422; an over-wide span is clamped to `min(effective max advance, 62)` days (the widest effective advance window across a pool), not rejected. The response echoes the effective window in `window: { from, to }` (`to` is the clamped bound) so a client can tell a silently-narrowed span from a genuine end of availability. Availability is real-time, so the response is not edge-cached. > **Note:** This endpoint requires the e-commerce feature to be enabled on your site.