Cancel a checkout session before payment processing starts or contract is created.

Common use cases:

• Customer abandonment: Track when customers decide not to proceed with purchase
• Duplicate removal: Cancel sessions when creating new ones with different terms
• Expiration handling: Clean up sessions that have reached their expiry time
• Fraud prevention: Cancel sessions flagged by risk management systems

Behavior & restrictions:

• Only 'open' sessions can be cancelled
• Sessions with payment_status 'pending' or 'paid' cannot be cancelled
• Provide optional cancellation context using standardized reason types:
  • customer_requested: When customer explicitly cancels
  • payment_failed: When payment attempt unsuccessful
  • expired: When session reaches expiration time
  • duplicate: When another session supersedes this one
  • fraudulent: When fraud risk detected
  • other: For any other reason (requires explanation)

Related endpoints:

• POST /checkout_session: Creates a session that may later be cancelled
• GET /checkout_session/{id}: Check session status before cancelling
• PUT /checkout_session/{id}: Alternative to cancellation when modifications needed

Errors include not-open sessions, disallowed payment status, and invalid status transitions.