fix(docs): keep copy button fixed in horizontally scrollable code blocks

[DraXx-Van]

Fixes #16293

Summary:

• Wrap each pre element in a .code-block-wrapper container
• Move copy button positioning to the wrapper instead of the scrollable pre element
• Update hover selector to target the wrapper

Result:
The copy button now remains fixed in the top-right corner while horizontally scrolling code blocks.

Tested:

• Copy button appears on hover
• Copy functionality still works
• Button remains fixed during horizontal scrolling
• Verified on actual Mongoose docs pages using browser overrides