Follow-up to https://github.com/modelcontextprotocol/ruby-sdk/pull/147.
According to the specification schema, `content` is not optional and therefore cannot be omitted:
```typescript
interface CallToolResult {
_meta?: { [key: string]: unknown };
content: ContentBlock[];
isError?: boolean;
structuredContent?: { [key: string]: unknown };
[key: string]: unknown;
}
```
https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult
Instead of `nil`, an empty array is set as the default value.
There may be a better value for `content`, but at the very least it should not be missing.
0034952df9