[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
```

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