[test-build] ContentElement allows image_url type without image payload

[huanghuoguoguo]

Background

While building the SDK test-build coverage branch, provider message conversion tests found that ContentElement allows inconsistent payloads.

Reproducer

message = Message(role='user', content=[ContentElement(type='image_url')])
message.get_content_platform_message_chain()

Expected: model validation should reject type='image_url' unless image_url is present, or conversion should return a structured validation error.

Actual: construction succeeds, and conversion later crashes when it assumes the image payload exists.

Risk

• Invalid provider messages can be accepted at SDK boundary and fail later in conversion.
• Plugin authors get a runtime exception far away from the bad input.
• Similar typed content variants may need required-payload validation as well.

Test Signal

Locked in test-build as an xfail(strict=True) contract test:

tests/api/entities/builtin/test_provider_message.py::test_message_image_url_content_should_validate_required_payload

No source fix is included in the test-build branch.

[huanghuoguoguo]

已在 #65（commit 9723b59）修复。

修复方式：Message / MessageChunk 转换 image_url content 时显式校验 image_url payload，缺失时抛出清晰的 ValueError，不再让无效内容静默通过或依赖 assert。

回归用例：tests/api/entities/builtin/test_provider_message.py::test_message_image_url_content_should_validate_required_payload。

测试：

• 对应 strict xfail 已取消，回归用例现在正常执行并通过。
• SDK：uv run pytest -> 269 passed, 10 warnings。
• LangBot master E2E：将本地 SDK editable 安装到 master worktree，WebUI 上传安装 qa/plugin-smoke，验证插件安装、Tool/Page 注册、Smoke Page iframe 渲染，以及 page-api /ping 返回 qa-plugin-smoke-page。