Standardize error response format in test Lambda function

[longwind48]

Problem

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

Error responses return HTTP-style objects:

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:

return {"message": f"Hello, {name}!", ...}

Other MCP Lambda functions (athena, cost_explorer, cur_analyst) consistently return plain dicts for both success and error:

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)