Restore Copy Button in OpenAPI Documentation

**Description**
The copy button is missing in the OpenAPI documentation due to style issues. This needs fixing to ensure users can easily copy code snippets and endpoints.

**Proposed Solution**
- Update CSS styles to ensure the visibility and functionality of the copy button.
- Test for consistent display across different browsers and devices.

**Acceptance Criteria**
- Copy button is visible and operational in all OpenAPI documentation sections.
- Consider adding tests to prevent future issues.

**Entry point for development**
https://github.com/diplodoc-platform/openapi-extension/blob/26d5501989eb04f04d89f8d8a2ef1cea258fdd4d/src/runtime/components/Response.tsx#L65