App Bridge resource picker test harness

Product: single select

type: 'product', multiple: false - picks at most one product.

Product: multi select

type: 'product', multiple: true - picks any number of products.

Product: variants shown

filter: {variants: true} - lets the merchant drill into a product and pick specific variants.

Variant: direct picker

type: 'variant' - a distinct top-level resource type, picks variants directly rather than via product drill-down.

Collection picker

type: 'collection' - picks one or more collections.

Product: pre-seeded selection

selectionIds with two made-up product gids - exercises pre-checking existing selections.

Product: filtered + action=select

action: 'select' (changes the title verb), filter: {draft: false, archived: false}, query: 'shirt' as initial search text.