Search does not look inside chat/session content — only matches session titles

### Summary

In the GitHub Copilot app, the **Search** feature (left sidebar / search box) does not search the **content of chats/sessions**. It appears to match only session **titles/names**, so I cannot find a conversation by something that was actually written or said *inside* it.

### Steps to reproduce

1. Open the GitHub Copilot app.
2. Make sure a past chat/session contains a specific string somewhere in its messages — e.g. a PR number (`15883012`), a file path (`src/auth/login.ts`), an error message, or a class/function name.
3. Open **Search** and type that string.
4. **Result:** "No results found" — even though that exact text exists inside one of my sessions.

### Expected behavior

Search should match text **inside chat messages** (both my prompts and Copilot's responses), not just the session title. Ideally it should return the matching session(s) with a short snippet of the match so I can jump straight to that conversation.

### Why it matters

I run many sessions, and they often have vague or duplicated auto-generated titles. The most reliable way I remember a conversation is by its **content** — a PR number, a file, an error string. Without content search I have to open sessions one by one and read them to find the right one.

### Additional context

Searching a PR number returns "No results found" although that number was discussed in one of my sessions (title-only matching). Making Search cover chat content would make it actually useful for finding past conversations.

首先定位 GitHub Copilot 应用的 Search 功能，并使用过去聊天中出现的 PR 编号或文件路径重现无结果的情况。当 Search 能在用户提示和 Copilot 响应中都找到匹配项，返回相关会话，并显示一小段匹配的摘要时，即表示完成。

由索引模型根据 Issue 内容生成。