docs: fix A2A SDK model imports

[FanouZeng-TT]

Description

The A2A integration guide's SDK import block still uses four module paths that
do not exist in the sample's pinned ucp-sdk==0.1.0 environment:

from ucp_sdk.models.schemas.shopping.checkout import Checkout, FulfillmentCheckout
from ucp_sdk.models.schemas.shopping.types.line_item import LineItem
from ucp_sdk.models.schemas.shopping.types.payment_response import PaymentResponse
from ucp_sdk.models.schemas.shopping.types.fulfillment_option_response import FulfillmentOptionResponse

Copying the block fails immediately with ModuleNotFoundError. The business
agent already imports these response models from the generated _resp modules.

Fix: align the guide with the working imports used by store.py, including
the fulfillment, buyer-consent, and discount checkout aliases described by the
dynamic type example above it.

Category (Required)

• Core Protocol: Changes to the UCP core protocol. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: Changes to UCP capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI, build, or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency or maintenance changes. (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. (Requires Maintainer approval)
• Community Health (.github): Community health file changes. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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.

Screenshots / Logs (if applicable)

The four stale modules fail to import in the pinned SDK environment. The full
replacement import set succeeds; full pre-commit and git diff --check pass.