feat: support capability extensions in codegen and cart-to-checkout conversion

[damaz91]

Description

This PR adds native capability extension support directly into the Python SDK codegen pipeline:

1. Request Variant Projection: Updates preprocess_schemas.py so that request variants (*_create_request.json, *_update_request.json, *_complete_request.json) are generated across all capability extensions in $defs (such as cart.json, fulfillment.json, discount.json, buyer_consent.json).
2. Generated Extension Models: Generates typed Pydantic models for capability extensions across shopping operations:
   • cart_create_request.Checkout: adds cart_id and relaxes line_items to optional.
   • fulfillment_create_request.Checkout / fulfillment_update_request.Checkout: adds fulfillment.
   • discount_create_request.Checkout / discount_create_request.Cart / discount_update_request.*: adds discounts.
   • buyer_consent_create_request.Checkout / buyer_consent_update_request.Checkout: adds buyer.
3. Cart Conversion Validator: Injects @model_validator(mode="after") (_enforce_cart_conversion) into cart_create_request.Checkout in postprocess_models.py to enforce that either cart_id or line_items is provided.
4. Postprocess AfterValidator Fix: Strips trailing whitespace and commas before inserting AfterValidator(...) into multiline Annotated aliases to prevent syntax errors during formatting.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• 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.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.