Fix PaymentHandler.display_order OpenAPI indentation

🔧 Type of Change

• Bug fix (non-breaking)

📝 Description

In openapi.agentic_checkout.yaml (both 2026-04-17 and unreleased), PaymentHandler.display_order is indented as a sibling of properties:/required: rather than nested inside properties:. Since display_order isn't a recognized OpenAPI/JSON-Schema keyword at the schema-object level, it's silently ignored there — and because PaymentHandler sets additionalProperties: false, a handler object that actually sets display_order (exactly what the field's own description invites merchants to do) fails schema validation with "unexpected field display_order".

The JSON Schema mirror (schema.agentic_checkout.json) already nests display_order correctly under properties in both versions — only the hand-maintained OpenAPI YAML had the indentation bug, so this brings the two representations back in sync.

🎯 Motivation and Context

Fixes/Addresses: #279

🧪 Testing

• Parsed both modified YAML files with a YAML loader and confirmed display_order now resolves under components.schemas.PaymentHandler.properties.
• Ran pnpm install && pnpm run validate:all — full validation suite passes (field types, descriptions, examples-against-schema across all versions), including the existing display_order example entries in examples/2026-04-17/examples.agentic_checkout.json and examples/unreleased/examples.agentic_checkout.json, which now validate against the corrected schema.

✅ Checklist

• My change follows the project's code style and conventions
• I have added a changelog entry file to changelog/unreleased/fix-payment-handler-display-order-indentation.md
• I have tested my changes locally
• This change does NOT require a SEP (it's minor/non-breaking)

🔍 Scope Verification

• This is a minor, straightforward change (confirm by checking this)

📚 Additional Notes

Two-line indentation fix, scoped only to the affected schema — no behavior/field changes beyond making display_order actually usable per its own documented intent.