fix(server): align Python sample server with UCP 2026-04-08

[koustubh25]

Summary

Aligns the Python sample merchant server with the official UCP 2026-04-08 release.

Dependency chain for April validation:

• Universal-Commerce-Protocol/python-sdk#28
• this PR
• Universal-Commerce-Protocol/conformance#41

This PR updates the Python server runtime to the April SDK/model layout so it can act as the reference merchant validation target for the April conformance suite.

Changes

• update Python server imports to the April SDK layout for extension checkout models:
  • ap2_mandate.dev.ucp.shopping
  • buyer_consent.dev.ucp.shopping
  • discount.dev.ucp.shopping
  • fulfillment.dev.ucp.shopping
• update discovery profile version/spec/schema URLs from 2026-01-23 to 2026-04-08
• fix checkout completion order construction for April SDK model requirements by:
  • carrying currency into the generated Order
  • serializing totals in the shape expected by the April models
• fix fulfillment total accumulation when option totals are wrapped by April scalar/root models

Why

After switching the Python sample server to the April SDK branch, the server still had January-era assumptions in extension imports and in checkout completion order construction. That caused the sample merchant server to fail when used as the validation target for the April conformance suite.

Validation

Validated with:

• python-sdk#28
• local conformance branch from Universal-Commerce-Protocol/conformance#41

Results:

• rest/python/server/integration_test.py OK (5 passed)
• updated Python sample server successfully served as the merchant target for the April conformance run backing conformance#41

Scope

Included here:

• runtime Python server compatibility with April SDK model layout and order serialization
• April discovery profile metadata for the Python sample server

Not included here:

• README/documentation refresh for remaining January examples
• Python SDK generator/model changes
• conformance test updates themselves