feat(types): echo checkout_session_id on PayResult

[dangazineu]

What

Add an optional checkout_session_id to PayResult (@codespar/types), and document the codespar_pay correlation in the contract doc.

When an agent drives a purchase as two steps — codespar_shop to reach ready_for_payment, then the payment tool to settle the pix_copia_e_cola — the PayResult it gets back today says nothing about which checkout it settled. Echoing the checkout_session_id gives a caller the join key to correlate the two calls without reaching into server-internal state.

Shape

export interface PayResult {
  // ...existing fields unchanged...
  pix_copy_paste?: string;
  checkout_session_id?: string; // NEW: the checkout this payment settled (join key)
  raw?: unknown;
}

Why it's safe (additive, v0 rule)

• New optional field; no existing field is removed, renamed, or re-typed.
• A direct payment with no originating checkout leaves it unset; consumers that ignore it are unaffected.
• The merchant pix_copia_e_cola the checkout carried is deliberately NOT echoed here — it is a payable bearer instrument and is kept out of results and logs (PII / Pix log-redaction).

Scope note (3-way edit)

PayResult is TypeScript-only today (no Python dataclass mirror in types.py), so this is a TS-types + backend-route change. Bringing codespar_pay's types to the TS/Python parity the shop types already have is tracked as separate follow-up work.

Verification

npm run typecheck and npm test green in packages/types (82 tests).

Draft pending the consuming backend change and a @codespar/types publish.