bug: Q CLI @<prompt> error messages are hard to understand.

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue

### Operating system

MacOS

### Expected behaviour

Q CLI tells me exactly what went wrong with my `@` invocation when I omit/malform prompt inputs.

### Actual behaviour

Q CLI gives me a raw error message when I omit/malform prompt inputs.
```
Error encountered while retrieving prompt:
{
"code": -32602,
"message": "MCP error -32602: Invalid arguments for prompt {prompt_name}: [\n {\n \"{input1}\": \"invalid_type\",\n \"expected\": \"object\",\n \"received\": \"undefined\",\n \"{input2}\": [],\n \"message\": \"Required\"\n }\n]"
}
```

### Steps to reproduce

Use `@` and omit inputs.

Start by reproducing the issue through the `@` entry point with omitted inputs and trace the prompt-retrieval error path. Done means Q CLI replaces the raw MCP validation output with an understandable message explaining which prompt inputs are missing or malformed.

Scritto dal modello di indicizzazione a partire dal testo della issue.