openrouter integration

### Describe the feature or problem you'd like to solve

setup openrouter api & choose their models

### Proposed solution

tried claude code with openrouter, got stuck while keep requestin/consuming the token.
i love to work with copilot-cli, openrouter can be used in copilot chat, i think it could be done in copilot-cli

or follow this https://openrouter.ai/docs/guides/coding-agents/claude-code-integration
thank you.

### Example prompts or workflows

_No response_

### Additional context

_No response_

从链接的 OpenRouter coding-agents 文档开始，然后检查 copilot-cli 现有的模型配置以及请求或令牌消耗流程。该 issue 未提供文件或测试入口；完成意味着具备受支持的 OpenRouter 设置、可选择的 OpenRouter 模型，并验证请求消耗不会出现所报告的循环。

由索引模型根据 Issue 内容生成。