docs(eino): add missing Role field in multimodal example

[ryougi-misaki]

Add the missing required parameter Role: schema.User to the example code

What type of PR is this?

docs: Documentation only changes

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docs(eino): 在多模态示例中补充缺失的 Role 字段

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
This PR fixes a critical omission in the multimodal chat example code. The schema.Message struct requires the Role field (e.g., schema.User) to be explicitly set, but it was missing in the documentation example. Without it, users would encounter runtime errors when trying to run the sample. This change adds the required Role: schema.User to ensure the example works correctly out of the box.

zh(optional):
本 PR 修复了多模态聊天示例代码中的一个关键遗漏。schema.Message 结构体必须显式设置 Role 字段（例如 schema.User），但文档示例中缺失了该字段，导致用户直接运行示例时会报错。本次修改补充了必要的 Role: schema.User，确保示例代码开箱即用。

(Optional) Which issue(s) this PR fixes:

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[GuangmingLuo]

@ryougi-misaki Please Sign CLA