fix: make profileId optional throughout PaymentSession surface

[Pradeep-kumar1202]

Summary

• Replaces the abandoned chore: Added the mandatory profile_id field #80 with the optional-profileId variant. profileId becomes String? = null everywhere in the payment-session flow; click-to-pay and authentication are deliberately excluded.

Why

• After chore: Added the mandatory profile_id field #80 was opened to make profileId mandatory across PaymentSession, product decided profileId should be inferred from the publishable key on the backend when not supplied. Forcing merchants to pass it is now a regression on integration ergonomics.