Standardize error response format in test Lambda function

## Problem

The test Lambda function (`src/lambda/mcp_servers/test/lambda_function.py`) uses inconsistent response formats:

**Error responses** return HTTP-style objects:
```python
return {"statusCode": 400, "body": json.dumps({"error": "message parameter is required"})}
return {"statusCode": 404, "body": json.dumps({"error": f"Unknown tool: {tool_name}", ...})}
```

**Success responses** return plain dicts:
```python
return {"message": f"Hello, {name}!", ...}
```

Other MCP Lambda functions (`athena`, `cost_explorer`, `cur_analyst`) consistently return plain dicts for both success and error:
```python
return {"error": str(e)} # errors
return {"results": [...]} # success
```

## Expected Behavior

The test Lambda should follow the same pattern as other MCP Lambda functions — return plain dicts without HTTP status codes wrapping.

## Scope

- Update error returns in `src/lambda/mcp_servers/test/lambda_function.py` to use `{"error": "..."}` format
- Remove `statusCode` and `body` wrapping from error responses
- Keep success response format as-is (already plain dict)