Unify copy actions for agent panel and markdown (#63884)

Before, it was not possible at all to copy the text from the agent
panel, only its markdown.
The PR breaks the existing ctrl/cmd-c binding there by using the same
approach markdown preview does: copy as text by default, allow to copy
as markdown via the context menu.

Before:


https://github.com/user-attachments/assets/2014f363-5167-4700-be86-9c0774d7e4fd

After:


https://github.com/user-attachments/assets/1dad253c-7a3b-4ca7-99c8-e8df8b3c3fd6


Release Notes:

- Changed agent panel to copy text with cmd/ctrl-c, as markdown preview
does. `markdown::CopyAsMarkdown` action is available still and can be
bound if needed.

docs/src/ai/agent-panel.md

@@ -79,7 +79,8 @@ The checkpoint button appears even if you interrupt the thread midway through an
 
 Right-click on any agent response in the thread view to access a context menu with the following actions:
 
-- **Copy Selection**: Copies the currently selected text as Markdown (available when text is selected).
+- **Copy**: Copies the currently selected text as plain text (available when text is selected).
+- **Copy as Markdown**: Copies the currently selected text as Markdown (available when text is selected).
 - **Copy This Agent Response**: Copies the full text of the agent response you right-clicked on.
 - **Scroll to Top / Scroll to Bottom**: Scrolls to the beginning or end of the thread, depending on your current position.
 - **Open Thread as Markdown**: Opens the entire thread as a Markdown file in a new tab.