Iteration 3: add checkout pay link card

[HimanM]

Summary

• parse successful checkout tool results into a structured order payload with payment link metadata
• render a dedicated pay-link card in chat while leaving incomplete checkout responses as plain assistant text
• document verification and behavior in the feature checklist

Verification

• python -m unittest tests.test_tool_arg_sanitizer tests.test_chat_route tests.test_provider_selection
• npm run lint
• npm run build
• Playwright fallback against http://localhost:3000 to verify incomplete checkout stays text-only in the live UI