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.

Inizia individuando la funzionalità Search dell’app GitHub Copilot e riproduci il caso senza risultati con un numero di PR o un percorso di file che compare in una chat precedente. Il lavoro è completato quando Search trova corrispondenze sia nei prompt dell’utente sia nelle risposte di Copilot, restituisce la sessione pertinente e mostra un breve snippet corrispondente.

Scritto dal modello di indicizzazione a partire dal testo della issue.