docs: use PostalAddress snake_case attribute

[FanouZeng-TT]

Description

The A2A extension guide's location-based tax example accesses
PostalAddress.addressRegion:

return state_rates.get(address.addressRegion, 0.05)

PostalAddress exposes the generated Python model field as
address_region; the camelCase attribute raises AttributeError when the
example is copied into an extension.

Fix: use address.address_region in the tax-rate example.

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)

PostalAddress(address_region="CA").address_region == "CA"; accessing
address.addressRegion raises AttributeError. Full pre-commit and
git diff --check pass.