Returns the laddering projections for the MPA — one item per available
purchases-projection document (typically compute and/or database; up to four
product lines as PS4C expands).

With no filters, returns all existing projection documents for the payer account in
stable product-line order (compute, then database, then any future lines in enum
order). When a filtered product line has no projection document, the response is an
empty items array (not 404). Partial projection documents return only the fields
available in storage.

404 is returned only when the payer account does not exist or the caller cannot
access it. A payer account that is not onboarded for PS4C still returns 200 with an
empty items array when no projection documents exist — use
GET /ps4commitments/v1/aws/payer-accounts (or get-by-id) for onboardingStatus.

Pagination: results are returned in stable product-line order (compute, then
database, then any future lines in enum order). Use maxResults to limit page size
(default 50, max 500). When more items remain, the response includes a non-null
pageToken; pass it unchanged on the next request with the same query parameters
(service, maxResults). rowCount is the number of items in this page. An invalid
pageToken returns 400 with code pagination_token_invalid; an expired token returns
400 with code pagination_token_expired.