test: verify card and instrument interoperability

[nkavian]

Summary

• Extend the interoperability peer for CARD, instrument payments, Stripe Seller offers, and explicit payment-status reads.
• Test pympp's actual protected-route response for pending settlement: HTTP 402 with the original 503 Problem Details body.
• Explain that upstream behavior and retaining the original transaction when recovering.

Verification

• make sync and make verify (formatting, lint, typing, tests, coverage, isolated package installation).
• Synthetic local payment and recovery exchanges; no upstream workaround or live payments.

Part of task 798. No SDK runtime behavior is changed.