feat: update conformance suite for UCP 2026-04-08

[koustubh25]

Summary

Updates conformance to the official UCP 2026-04-08 release.

Dependency chain for April validation:

• Universal-Commerce-Protocol/python-sdk#28
• Universal-Commerce-Protocol/samples#91
• this PR

This PR removes stale 2026-01-23 protocol expectations from the suite, updates official ucp.dev spec/schema references to April, and adapts the tests to the April Python SDK model layout where the existing suite assumed January-era imports and scalar wrappers.

Changes

• update hard-coded UCP version expectations from 2026-01-23 to 2026-04-08
• update official spec/schema URLs to https://ucp.dev/2026-04-08/...
• update ReverseDomainName imports for the April Python SDK layout
• make conformance assertions compatible with April SDK RootModel wrappers used for Amount, SignedAmount, and Totals
• update the shopping agent profile fixture to advertise April capability metadata

Why

The official latest UCP spec release is 2026-04-08, but conformance still contains January-era protocol pins and SDK assumptions. That causes the suite to drift from the current published spec and from the April Python SDK update.

This PR keeps the scope limited to conformance-suite April alignment. Merchant-server fixes remain in samples#91, and SDK model updates remain in python-sdk#28.

Validation

Validated against:

• official spec overview: https://ucp.dev/latest/specification/overview/
• official April snapshot: https://ucp.dev/2026-04-08/specification/overview/
• python-sdk#28
• samples#91
• this branch

Serial test run result against the local April validation target:

• protocol_test.py OK
• validation_test.py OK
• fulfillment_test.py OK
• business_logic_test.py OK
• checkout_lifecycle_test.py OK
• idempotency_test.py OK
• order_test.py OK
• webhook_test.py OK
• simulation_url_security_test.py OK
• ap2_test.py OK
• binding_test.py OK
• card_credential_test.py OK

test_discovery_urls remains skipped because the remote schema publication check is not yet live on ucp.dev.

Scope

Included here:

• conformance version/spec/schema alignment
• test-harness compatibility with April SDK model shapes

Explicitly not included here:

• merchant-server implementation fixes in samples
• SDK generator or model-shape changes in python-sdk

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