feat(copilot): add Export Chat as Markdown option to session menu (#13070)

## Summary

Adds an **Export chat** option to the ⋯ dropdown on each CoPilot sidebar
session. Clicking it fetches the full conversation and instantly
downloads a `.md` file in the browser.

## Changes

### New file: `helpers/exportChatAsMarkdown.ts`
- Formats user/assistant turns as `## User` / `## Assistant` markdown
headers
- Renders tool calls inline as `> 🔧 tool_name(args)` blockquotes — shows
*what* the agent did without dumping raw JSON result blobs
- Skips `role === "tool"` result messages
- Sanitizes filename; uses Blob + anchor download pattern (same as
`downloadArtifact.ts`)
- Filename format: `chat-<title>-<YYYY-MM-DD>.md`

### Modified: `ChatSidebar.tsx`
- Adds **Export chat** `DropdownMenuItem` (with `DownloadSimple` icon)
between Rename and Delete chat
- Fetches full session on demand via `getV2GetSession` with `limit:
2000`
- Shows success/error toast feedback

## Testing
1. Open CoPilot, hover a session in the sidebar → click ⋯
2. **Export chat** appears between Rename and Delete chat
3. Clicking it downloads `chat-<title>-<date>.md` immediately
4. File contains `## User` / `## Assistant` sections; tool calls show as
`> 🔧 tool_name(...)`
5. Error toast appears if the session fetch fails

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Co-authored-by: Toran Bruce Richards <22963551+Torantulino@users.noreply.github.com>
Co-authored-by: abhi1992002 <abhimanyu1992002@gmail.com>