MCP enabled but no registry servers or tools discovered (no registry UI)

**Title**
Copilot for Xcode: MCP enabled but always discovers 0 servers / 0 tools on GitHub.com (no registry UI)

**Describe the bug**
GitHub Copilot for Xcode shows MCP as enabled and triggers MCP refresh, but it never discovers any MCP servers or tools `(Servers: 0, Total tools: 0)`. Our org has a private MCP registry configured in allow “registry-only” mode, with GitHub's MCP server in the allowlist. The same registry works on github.com browser Copilot chats and in other IDEs (VS Code, IntelliJ), but not in Copilot for Xcode.

In Settings → Tools → MCP:
- MCP is enabled
- “Available MCP Tools” is empty
- UI consistently reports “Servers: 0, Total tools: 0”

The only editable MCP configuration is a local `mcp.json` file (via “Edit Config”) containing a `servers` object. There is no UI to view/configure an MCP registry base URL for GitHub.com (non-GHES) accounts. Logs show refresh succeeds but results are always 0 servers / 0 tools, with no MCP errors.

**Versions**
- Copilot for Xcode: 0.46.0
- Xcode: 26.1.1 (17B100)
- macOS: 26.2 (Tahoe)

**Steps to reproduce**
1. Install Copilot for Xcode 0.46.0
2. Sign in with a GitHub.com Copilot Business account (non-GHES)
3. Open Copilot for Xcode Settings → Tools → MCP
4. Confirm MCP is enabled and “Available MCP Tools” is empty
5. Click “Edit Config”; observe default `mcp.json` contains `"servers": {}`
6. Enable Verbose Logging
7. Trigger an MCP refresh (activation or settings change)
8. Observe logs: refresh succeeds but servers/tools remain 0

**Screenshots**
- Tools → MCP showing MCP enabled and “Available MCP Tools” empty (“Servers: 0, Total tools: 0”)

- `mcp.json` opened via “Edit Config” showing `"servers": {}`

- Tools → Built-In showing built-in tools present (tools UI otherwise works)

**Logs**
```
Copilot policy updated: customAgent=true, mcp=true, subagent=true
Refreshing MCP server tools…
Refreshed MCP tools - Servers: 0, Total tools: 0, Server names: []
```

**Additional context**
- Occurs on GitHub.com Copilot Business (not GHES)
- No MCP-related errors/warnings during refresh; result is consistently 0 servers / 0 tools
- Logs also show an `Apple Events permission error on activation: ‘Not authorized to send Apple events to System Events.’ (NSAppleScriptErrorNumber=-1743).` seems unrelated