clarify extension inputs in create checkout

[karangoel16]

Summary

• Render extension-owned top-level Create Checkout inputs (discounts, fulfillment) in the generated inputs table when those extensions are included
• Add a Create Checkout extension-input summary for Discount, Fulfillment, Buyer Consent, Payment Terms, and Split Payments
• Clarify in the Fulfillment extension docs that Create and Update requests may carry buyer-provided fulfillment selections/identifiers
• Update MCP checkout parameter descriptions/docs to call out negotiated fulfillment request behavior

Validation

• uv run python -m py_compile main.py
• python3 -m json.tool source/services/shopping/mcp.openrpc.json >/dev/null
• git diff --check
• DOCS_MODE=spec UCP_BUILD_VERSION=draft uv run mkdocs build -d /tmp/ucp-mkdocs-check
• ./scripts/build_local.sh --draft-only did not complete because local ucp-schema is 1.3.0 while crates.io has 1.4.1
• Strict MkDocs build still fails on existing Cairo/native library warnings from cairosvg in this environment