feat!: use type aliases instead of RootModel for root schemas

[damaz91]

Description

This PR modifies the codegen configuration to avoid generating Pydantic RootModel classes for root schemas (schemas that don't define an object at the root, like Amount). Instead, it generates them as PEP 695 TypeAliasTypes combined with Annotated for validation.

This resolves the issue where generated Python SDK classes had fields called root (e.g., Amount.root).

The SDK has been regenerated using UCP schema version 2026-04-08.

Type of change

• 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?

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

Breaking Changes / Removal Justification

Switching from RootModel classes to type aliases is a breaking change because:

1. Root types can no longer be instantiated as classes (e.g., PaymentHandler(...) is invalid). Clients must use the underlying primitive types directly.
2. The .root attribute is no longer available on these types (e.g., checkout_req.fulfillment.root will raise AttributeError).

Downstream conformance tests have been updated in a separate PR to handle this compatibility.

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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 (compiled successfully, conformance tests verified with updates)
• Any dependent changes have been merged and published in downstream modules