Support Google Agent Platform (Vertex AI) as a BYOK provider

I saw the #711 on this topic but:

> fixed/answered/superseded https://github.com/github/copilot-sdk/issues/711#issuecomment-4626626207

Well Agent Platform (a.k.a. Vertex AI) is still not supported ...

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Re-opening it as it is impacting us at the moment and we'd like to avoid switching to other tools that already supports it (e.g. Claude)

首先阅读 issue #711 中之前的讨论，并将其与此请求进行比较：将 Google Agent Platform (Vertex AI) 作为 BYOK 提供商。Payload 中没有列出文件、入口点或测试；完成意味着支持 Vertex AI，且不要求用户切换工具。

由索引模型根据 Issue 内容生成。