docs: wrap checkout input objects in a checkout object

[dvisockas]

Description

Wrapping Checkout MCP request docs in a checkout object.
Before:

{
  "id": "checkout_id",
  "line_items": {}
}

After:

{
  "id": "checkout_id",
  "checkout": {
    "line_items": {}
  }
}

Type of change

Documentation update

---

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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[lemonmade]

This should stay a top-level field, although we also currently require it in checkout.id for update requests. @richmolj I feel we should update checkout.id to omit on update, since the transports all put ID out-of-band; what do you think?

[dvisockas]

Fair point, changed