[Bug]: Can't find agent card with endpoint /.well-known/agent.json

### What happened?

ADK A2A server by default expose agent card at /.well-known/agent.json endpoint.

The inspector keeps looking for /.well-known/agent-card.json.

At least support overriding of acceptable endpoint.

### Relevant log output

```shell
Error: An internal server error occurred: HTTP Error 404: Failed to fetch agent card from https://planner-agent-550681616.us-central1.run.app/.well-known/agent-card.json: Client error '404 Not Found' for url 'https://planner-agent-550681616.us-central1.run.app/.well-known/agent-card.json' For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The inspector is looking for the agent card at the wrong endpoint. Find where the endpoint URL is constructed, likely in a configuration or fetch call. The fix is to change the default from '/.well-known/agent-card.json' to '/.well-known/agent.json' or make it configurable. Check for related tests to ensure the change works.

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