fix: support ucp envelope in discovery profile tests

[antomtech]

Description

The UCP specification (2026-01-23 / 2026-04-08) wraps the .well-known/ucp discovery response under a top-level "ucp" key. The current test code assumes services is always at the top level of the JSON response, which causes test failures against compliant server implementations that use the envelope format.

This PR adds backward-compatible envelope unwrapping:

• If the response contains a "ucp" key, it is used as the root for field access.
• If not, the original top-level object is used (maintaining backward compatibility).

Files changed

• integration_test_utils.py: Updated discovery profile parsing to unwrap the ucp envelope before extracting shopping service endpoints.
• protocol_test.py: Updated all discovery-related tests to unwrap the ucp envelope when validating profile structure, service bindings, and version negotiation.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

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[antomtech]

I have read the CLA Document and I hereby sign the CLA

[antomtech]

@googlebot I signed it.