Wire onCopy/onError on default code-block copy controls

[ShobhanKarthish]

Summary

• Extend ControlsConfig with CopyControlConfig so controls.code.copy / controls.mermaid.copy can be { onCopy, onError } (still accepts boolean).
• Thread those callbacks into the default-rendered CodeBlockCopyButton via getCopyCallbacks.
• Add unit and integration coverage for callback extraction and default control wiring.

Hosts using default controls can now surface copy failures and hook success events without re-owning the full code composition:

<Streamdown
  controls={{
    code: {
      copy: {
        onCopy: () => announce("Copied"),
        onError: (error) => toast.error(error.message),
      },
    },
  }}
>
  {markdown}
</Streamdown>

Closes #557

Test plan

• vitest run controls.test.ts show-controls.test.tsx
• vitest run code-block.test.tsx
• Click default code copy button with controls.code.copy.onCopy and confirm callback fires
• Confirm clipboard-unavailable path calls onError
• Confirm copy: false still hides the button and object config still shows it

[vercel]

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