docs(a2a): fix integration guide spec links

[FanouZeng-TT]

Description

The A2A UCP integration guide's merchant profile example still points two
spec fields at the old nested shopping documentation paths:

"spec": "https://ucp.dev/2026-01-23/specification/shopping"
"spec": "https://ucp.dev/2026-01-23/specification/shopping/checkout"

Both URLs now return 404. The same-version overview and checkout paths are
reachable, and the checkout schema URL in the example is already valid.

Fix: update only the two documentation spec links to the reachable
same-version paths while leaving the sample profile version and schema URL
unchanged.

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.

Screenshots / Logs (if applicable)

• parsed the guide's merchant profile JSON block successfully
• verified the updated service, checkout, and checkout schema URLs return HTTP 200
• uvx pre-commit run --all-files
• git diff --check