Show all accounts used by AI providers and MCP servers in the profile menu

## Problem

The Copilot profile menu currently shows the GitHub account and ChatGPT sign-in, but it does not provide a way to see the other accounts used by AI-related features. The existing Accounts menu can show some of these accounts, but that leaves the Copilot profile menu incomplete and makes it difficult to understand which identities are active for AI features.

This matters because:

- MCP servers can depend on authenticated accounts.
- BYOK providers such as Azure AI Foundry depend on accounts.
- Microsoft sign-in for inference will add another account-backed flow.

## Expected

Provide a coherent account story in the Copilot profile menu that displays all accounts currently used by AI providers, MCP servers, and inference, with a path to inspect or manage them. The UI should scale beyond GitHub and ChatGPT rather than adding one-off entries for each provider.

The existing Accounts menu may be useful prior art, though its current presentation should not necessarily be copied as-is.