Plugin-bundled local MCP servers (stdio) show in UI but cannot be connected/enabled

## Description

Plugin-bundled local stdio MCP servers appear in the Desktop plugin settings UI under "Tools and data sources this plugin connects to" but there is no button, toggle, or mechanism to connect/enable them. They remain permanently disconnected.

## Reproduction

1. Create a plugin with a `.mcp.json` that defines a local stdio server:
```json
{
"mcpServers": {
"ow-server": {
"command": "npx",
"args": ["-y", "@oriensworld/ow-server", "--stdio"]
}
}
}
```
2. Install the plugin via zip upload or marketplace
3. Open plugin settings in Desktop — the MCP server is listed but shows no way to connect it
4. Invoke the plugin's skill that depends on the MCP server — reports "MCP server isn't connected"

## Affects official plugins too

The same behavior occurs with Anthropic's official `pdf-viewer` plugin (`@modelcontextprotocol/server-pdf --stdio`). Its connector is listed but cannot be activated either.

## Expected behavior

Local stdio MCP servers bundled in plugins should either:
- Auto-start when the plugin is enabled (like they do in Claude Code)
- Or provide a visible "Connect" / "Enable" toggle in the UI

## Workaround

Manually adding the MCP server to `claude_desktop_config.json` works:
```json
{
"mcpServers": {
"ow-server": {
"command": "npx",
"args": ["-y", "@oriensworld/ow-server", "--stdio"]
}
}
}
```

## Environment

- Claude Desktop: 1.18286.0 (259c3f) built 2026-07-02T07:11:03.000Z
- OS: Windows 11 (10.0.26200)
- Node.js: v24.11.1
- Plugin format: `.claude-plugin/plugin.json` manifest with `.mcp.json` at plugin root
- Tested via: zip upload and marketplace install (`oriensworld/ow-marketplace`)