fix: Do not panic when reading malformed properties

[gmlewis]

Background: I was trying to run the command:

• mcphost -m google:gemini-2.5-pro

with this ~/.mcp.json file:

{
  "mcpServers": {
    "desktop-commander": {
      "command": "npx",
      "args": [
        "-y",
        "@smithery/cli@latest",
        "run",
        "@wonderwhy-er/desktop-commander"
      ]
    }
  }
}

and set my GOOGLE_API_KEY appropriately.

mcphost started properly and the /tools and /servers commands worked fine.
However, when I typed in "How many ebooks are in my Downloads folder?" it would panic within the propertyToGoogleSchema method because when "name" was "value", the resulting map was empty (although the propertyToGoogleSchema method assumes that the "type" key exists.

This PR fixes the panic on malformed schema.Properties.