fix(snapshot): suppress unavailable offer fields

[intel352]

Summary

• clear price, seller, shipping, total, and Prime fields when Amazon reports an item unavailable
• add regression coverage for unavailable pages that still contain stale offer DOM fields

Verification

• GOWORK=off go test ./internal/snapshot -run 'TestExtractAmazonUnavailableProduct' -count=1
• GOWORK=off go test ./...
• live capture: B0CMVPN6GL now returns availability only with no transactional fields
• live capture: B0DL7CKRJ5 still returns third-party seller/price/shipping fields