MCP: switch from JSON TextContent to StructuredContent

[AkihiroSuda]

lima/pkg/mcp/toolset/shell.go

Lines 48 to 54 in c85e72c

	resJ, err := json.Marshal(res)
	if err != nil {
	return nil, nil, err
	}
	return &mcp.CallToolResult{
	Content: []mcp.Content{&mcp.TextContent{Text: string(resJ)}},
	}, nil, nil

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content says:

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block

However, probably we do not need to retain the JSON TextContent block if StructuredContent is supported by well-known agents (Gemini, Claude, Codex, etc.)