Omit Google Pay billingAddressParameters when billing is not required

[Max Harrison (maxharrison)]

Stacked on #985. Review that first.

buildPaymentRequest always sent billingAddressParameters, even when billingAddressRequired was false:

"billingAddressRequired": false,
"billingAddressParameters": { "format": "FULL", "phoneNumberRequired": false }

Google ignores the parameters when billingAddressRequired is false, so the payment sheet does not change. The request still claimed a FULL address format that we never asked for, which is misleading in logs and in support tickets.

Omitting the field makes the request state only what it means, and makes billing-disabled.json identical to the Android fixture.

Not a breaking change for merchants. The only visible effect is one fewer field on the wire, shown here as the fixture diff.

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[Max Harrison (maxharrison)]

Reopened as #987 after the stack was reordered. GitHub auto-closed this one as merged into its former stack base; nothing landed on master.