fix!: Mark checkout id as deprecated_required_to_omit for update requests

[raginpirate]

Dependency on Universal-Commerce-Protocol/ucp-schema#6

Description

This change simplifies the checkout request schema by marking the id field as deprecated from all request types. Today we do this correctly for complete, but for update we accept a checkout id in both the transport layer as well as the checkout object being provided. This was a miss dating back to the 2026-01-11 release.

Type of change

• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

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.

Its very clear why this is needed; redundant context being sent between businesses and platforms.

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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

[knightlin-shopify]

LGTM to omit the id field for all the request. The id should be specified outside of checkout object (REST will be on the URL, MCP will be on parent object).