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