Add copy buttons for assistant message text blocks

[SheferKagan]

Summary

Add a dedicated copy button for individual copyable sections inside assistant chat messages.

Today the chat UI has a message-level copy action, but users often only want to copy one specific block from an assistant response: a command, code snippet, config snippet, prompt, JSON payload, quoted/styled text box, etc.

Desired behavior

When an assistant message renders a distinct copyable block, that block should expose its own copy button.

Examples of copyable blocks:

• fenced markdown code blocks
• command snippets
• JSON/config snippets
• styled text boxes/callouts that contain copyable text
• quoted/styled blocks where the intent is to copy only that section

The button should copy only the contents of that block, not the entire assistant message.

Example

Assistant response:

Run this:

[code block]
npm run build
[Copy]

Then check the output.

Clicking the block-level copy button should copy only:

npm run build

not the full assistant response.

UX notes

• Keep the existing whole-response copy action.
• Add the section-level copy affordance on or near the block itself, likely top-right.
• Use a clear icon + tooltip, consistent with existing chat copy buttons.
• Provide copied feedback, e.g. icon changes/checkmark or tooltip text changes to Copied.
• Avoid adding visual noise to normal paragraphs.
• The button should work for both code blocks and styled/callout-like text boxes.
• Long blocks should keep the copy button visible and aligned.
• Copy should preserve the plain text contents of the block.

Acceptance criteria

• Code/fenced blocks in assistant messages have a dedicated copy button.
• Styled text boxes/callouts in assistant messages can expose a dedicated copy button when they contain copyable text.
• Clicking a block copy button copies only that block's text.
• Message-level copy still copies the whole assistant message.
• Copy feedback is visible.
• Layout remains stable for long blocks and narrow widths.
• No copy button is added to normal prose paragraphs.