Support stdio transport server on the session/new request in the acp mode.

### Describe the bug

Agent Client Protocol says [All agents MUST support this transport](https://agentclientprotocol.com/protocol/v1/schema#mcpserver) but Copilot CLI has Rejected these stdio servers.

### Affected version

GitHub Copilot CLI 1.0.63

### Steps to reproduce the behavior

1. Create ACP Client and it connect to Copilot CLI
2. request a `session/new` request with `mcpServers` config include the stdio transport servers.
3. Copilot CLI will rejected these stdio servers. (I discovered Copilot CLI says reject in the process log)

### Expected behavior

These will launch correctly stdio transport MCP Servers via `session/new` request

### Additional context

Currently workaround:
mcpServers config pass to copilot cli via `--additional-mcp-config=JSON` args, these works fine.

Is that the behavior correct?

thanks.

Start at the ACP mode handler for the session/new request and trace how its mcpServers configuration is processed, comparing it with the --additional-mcp-config path that already works. Reproduce the request with a stdio transport server and verify that the server launches successfully through session/new without being rejected.

Written by the indexing model from the issue text.