Add on-demand connection mode for ephemeral MCP servers

[Copilot]

Some MCP servers (e.g., pdf-reader) are designed to handle one request and exit, causing "Connection closed" errors when MCPHub tries to maintain persistent connections.

Changes

Core behavior:

• Added connectionMode field to server configuration: "persistent" (default) | "on-demand"
• On-demand servers connect only during tool invocation, disconnect immediately after
• During initialization, on-demand servers connect briefly to discover tools, then disconnect

Implementation details:

• connectOnDemandServer() / disconnectOnDemandServer() helper functions manage lifecycle
• Tool invocation wrapped in try-finally to guarantee cleanup even on errors
• Smart routing updated to include disconnected on-demand servers in available tools
• Comprehensive test coverage (9 new tests) for connection lifecycle and error cases

Usage

{
  "mcpServers": {
    "pdf-reader": {
      "command": "npx",
      "args": ["-y", "pdf-mcp-server"],
      "connectionMode": "on-demand"
    }
  }
}

Backward compatible—servers default to "persistent" mode.

Original prompt

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This section details on the original issue you should resolve

<issue_title>MCP services are divided into long-term and short-term when they are added</issue_title>
<issue_description>I have an mcp service mcp/pdf-reader, docker runs, but this service does not support long-term running, it is one-time, and it will give an error when running:
Failed to connect: McpError: MCP error -32000: Connection closed
at Client._onclose (file:///app/node_modules/.pnpm/@modelcontextprotocol[email protected]/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:109:23)
at _transport.onclose (file:///app/node_modules/.pnpm/@modelcontextprotocol[email protected]/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:76:18)
at ChildProcess. (file:///app/node_modules/.pnpm/@modelcontextprotocol[email protected]/node_modules/@modelcontextprotocol/sdk/dist/esm/client/stdio.js:95:77)
at ChildProcess.emit (node:events:519:28)
at maybeClose (node:internal/child_process:1101:16)
at ChildProcess._handle.onexit (node:internal/child_process:304:5)

In principle, the popularity of this feature request is not that high, but it is only a situation that I have encountered when I use it personally. Hope there is this feature, but it is not necessary, thanks</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes MCP services are divided into long-term and short-term when they are added #399

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