fix: Remove 'checkout' wrapper from doc examples

[richmolj]

Description

#141 incorrectly added a checkout wrapper key in the documentation examples. This does not reflect the schema or what we believe to be correct, so fixing those examples.

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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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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[richmolj]

#216 also has this fix

[igrigorik]

Closing, resolved via #216.

[igrigorik]

LGTM — this is my bug from #141 🤦🏻

Looks like when I retrofitted the MCP structuredContent envelope onto the existing examples, I kept the "checkout"/"cart" wrapper key that was previously part of the old raw JSON-RPC result shape. Should've peeled it off on when adding the new layer.