Create checkout session

Creates a checkout session and returns the URL the client should redirect the user to.

Use this as the final "continue to payment" step from basket/checkout pages.

What this endpoint checks before creating a session:

• listing availability and purchasability
• seller authorization constraints
• basket consistency at checkout time

Response behavior:

• On success: returns a checkout URL.
• On failure: returns listing-level validation errors for unavailable/invalid items.

Heads up:

• Requires authentication.