Preselect the first payment option when the buyer holds no choice yet

[boo-code]

Questions	Answers
Description?	On a fresh guest checkout no payment option is selected, so every first-time buyer faces one extra mandatory decision plus a tap, and the "Please select a payment method" stop is reachable at submit. Baymard's payment-selection research (https://baymard.com/blog/payment-method-selection) recommends preselecting the most popular method so "the majority of users can proceed without a deliberate choice". This defaults the selection server-side to the first available payment option when the buyer holds no valid persisted choice: resolvePersistedSelection() falls back to resolveDefaultSelection(), which returns the first option in merchant order (payment modules are ordered by position, so the first option is the shop's preferred method). No client-side change is needed — the template already renders a server-selected option as checked, and the opcPaymentMethodsUpdated listener already confirms it and opens its panel, exactly as for a persisted selection; a buyer's own persisted choice still takes priority, and an invalidated persisted choice now also falls back to the first option instead of an empty selection. Raised in the discussion on #96.
Type?	improvement
BC breaks?	no
Deprecations?	no
Fixed ticket?
How to test?	vendor/bin/phpunit -c tests/php/phpunit.xml --filter OpcPaymentMethodsHandlerTest (new test: with an empty cookie the response carries the first option's module and selection key; existing persisted-selection test unchanged). On a 9.2 shop, fresh guest checkout: once the address is complete the first payment option renders preselected with its panel open, and the order can be placed without ever clicking the payment list — verified end-to-end (order placed with the preselected method, payment recorded as Bank Wire). Selecting a different option and reloading still restores the buyer's own choice.

[ps-jarvis]

Hello @boo-code!

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