fix!: Update validation test to follow documentation.

[semenovm]

Description

Please include a summary of the changes and the related issue. Please also
include relevant motivation and context.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

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Is this a Breaking Change or Removal?

If you checked "Breaking change" above, or if you are removing any schema
files or fields:

• I have added ! to my PR title (e.g., feat!: remove field).
• I have added justification below.

Breaking Changes / Removal Justification

This change makes python and likely ts example to fail validation tests.
This change makes tests follow the protocol documentation and spec, python implementation is incorrect and must be updated to pass the new test.

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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
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Update validation tests to:

1. Expect soft-fail when supported following https://ucp.dev/specification/checkout/#error-handling
2. Use correct error message format for hard fails https://ucp.dev/specification/checkout-rest/#error-responses