fix(ep-commerce): provide FormProvider from EP Product Provider

[mzaintariq]

What does this MR do?

Mounts a react-hook-form FormProvider inside EPProductProvider, matching commerce Product Box’s contract. Without it, descendants that write selection via form context (SelectedLocationSlug, variants, quantity for ATC) silently no-op when the PDP uses EP Product Provider instead of Product Box.

Changes

• Wrap provider children in FormProvider from useForm()
• Reset form state when the product id changes
• Doc comment clarifying parity with commerce ProductProvider

Design decisions

• EP Product Provider is the SSR-friendly product context; location picker / ATC already assume form context (“must be inside a Product Box”)
• Product Box already provides FormProvider; this closes the gap for compositions that only use EP Product Provider
• Cart drawer qty/remove do not depend on this — they use cart line data, not PDP form state

Testing

• Manual: PDP with EP Product Provider + location radios — selecting a location updates selection / URL and ATC receives location
• No change expected for Product Box–based PDPs (nested form providers remain the existing composition)

Reviewer notes

• Independent of cart drawer designer-error work; no dependency on that PR
• Follow-up found while testing multilocation ATC → cart; not required for cart-error DataProviders
• Consider a small unit/integration test later if we want FormProvider presence covered in CI

[field123]

Correct premise — there's no FormProvider anywhere in elastic-path today, so the PDP → ATC wiring really is dead. Keep it.

The reset is the problem (inline). Also worth adding the test now, not as a follow-up: the infra exists in registerEPAddToCartButton.test.tsx, and a child that setValues in a mount effect would catch this.

[field123]

reset() fires on mount, and child effects run before parent effects. EPStockProvider.tsx:232-243 sets SelectedLocationSlug from ?location= with deps [form] (stable ref in RHF 7.46.2), so it runs once — then this wipes it and never re-runs. Deep-linked location is lost and ATC posts with no locationId.

Suggest dropping the effect and mirroring commerce (contexts.tsx:26-32) — fresh form at render, before any child effect:

function ProductFormScope({ children }: { children: React.ReactNode }) {
  const methods = useForm();
  return <FormProvider {...methods}>{children}</FormProvider>;
}

// <ProductFormScope key={dataProduct?.id}>…</ProductFormScope>

[field123]

"No change for Product Box–based PDPs" isn't established — compositions where form writers straddle this boundary (commerce Product Quantity outside, EP ATC inside) now split across two form contexts. Worth checking the real storefront pages before merge.