fix(a2a): map string product images to line item image_url

[FanouZeng-TT]

Description

_get_line_item in a2a/business_agent/src/business_agent/store.py only
handles product images inside a list guard, then checks the same value
for str again inside that guard:

if isinstance(product.image, list):
    if isinstance(product.image, str):
        image_url = product.image

The inner branch is unreachable, so a legal single-string Product.image
never reaches the line item's item.image_url. Product.image is declared
as str | list[str] | list[ImageObject] | None, so all three non-null
shapes should project a URL onto the checkout line item.

Fix: handle a plain string first, then a non-empty list (first element
kept: a string as-is, an ImageObject mapped to .url). List behavior is
unchanged; the single-string shape is restored.

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)

• pre-fix: single-string image produces item.image_url == None
• post-fix: single string, list[str], list[ImageObject], and None all behave correctly
• uv run --group dev ruff check and ruff format --check
• uvx pre-commit run --all-files
• git diff --check