Add params passthrough to createCheckoutSession

[advaitpaliwal]

Summary

• createCheckoutSession previously only exposed a fixed set of Stripe Checkout parameters, forcing users to bypass the component entirely for common needs
• Adds an optional params field that accepts any Stripe.Checkout.SessionCreateParams and merges them into the session config
• mode is excluded from overrides to prevent inconsistency with the component's internal routing
• Fully backward compatible — existing code works unchanged

Use cases unlocked

// Promotion codes
await stripeClient.createCheckoutSession(ctx, {
  priceId: "price_...",
  mode: "subscription",
  successUrl: "...",
  cancelUrl: "...",
  params: {
    allow_promotion_codes: true,
  },
});

// Multiple line items
await stripeClient.createCheckoutSession(ctx, {
  priceId: "price_...",
  mode: "payment",
  successUrl: "...",
  cancelUrl: "...",
  params: {
    line_items: [
      { price: "price_basic", quantity: 1 },
      { price: "price_addon", quantity: 2 },
    ],
  },
});

// Billing address + custom fields
await stripeClient.createCheckoutSession(ctx, {
  priceId: "price_...",
  mode: "payment",
  successUrl: "...",
  cancelUrl: "...",
  params: {
    billing_address_collection: "required",
    phone_number_collection: { enabled: true },
  },
});

Test plan

• Verify existing createCheckoutSession calls without params work unchanged
• Verify params values override constructed defaults (e.g. custom line_items)
• Verify mode cannot be overridden via params
• Verify allow_promotion_codes, billing_address_collection, and other Stripe params work when passed

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