fix: send the type of payment confirmation models

[mokevnin]

Fixes #10.

A confirmation built from the SDK models was sent without its required type:

Yoomoney::PaymentCreateParams::Confirmation::ConfirmationDataRedirect.new(return_url: "https://example.com")
# before: {"return_url":"https://example.com"}
# after:  {"return_url":"https://example.com","type":"redirect"}

• lib: each variant of PaymentCreateParams::Confirmation declares required :type, const: :<value>, so it serializes its type by itself. The union has discriminator :type.
• rbi / sig: the variant constructors take locale: (inherited from ConfirmationData) and type:, which defaults to the variant's value; type is typed on each variant.
• test: covers each variant's request body (it fails without the lib change) and picking a variant from a hash by type.

The spec side is Hexlet/yoomoney-api#2.

rake lint (rubocop, Sorbet, Steep) and rake test (99 runs, 0 failures) pass.

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