aws-diagram-mcp-server: Gemini backend rejects tool registration due to missing schema type

### What happened?

Same issue as https://github.com/awslabs/mcp/issues/661

### What did you expect to happen?

Be able to generate an AWS diagram

### Client information

<details>

```console
$ gemini /about
# paste output here
```

</details>


### Login information

_No response_

### Anything else we need to know?

_No response_