feat: add checkout session params + expand discount data in webhook

[alim888aa]

Closes issue #35

Adds optional parameters to createCheckoutSession:

• allowPromotionCodes — enable promo code input on hosted checkout
• paymentMethodTypes — explicit payment method control
• paymentMethodOptions — method-specific config
• discounts — server-applied coupons/promotion codes
• locale — checkout page language
• currency — currency override

Also expands discount/promotion code data in the checkout.session.completed webhook handler via stripe.checkout.sessions.retrieve() with expand: ["discounts.promotion_code"], so custom handlers can read the promotion code string directly without additional API calls. This follows the existing pattern where processEvent already retrieves subscriptions/invoices for subscription mode.

All new parameters are optional. Existing behavior is unchanged. Includes a runtime guard for the mutually exclusive allowPromotionCodes/discounts params.

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