Add copy-path tool (#27371)

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Release Notes:

- N/A

crates/assistant_tools/src/copy_path_tool/description.md

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+Copies a file or directory in the project, and returns confirmation that the copy succeeded.
+Directory contents will be copied recursively (like `cp -r`).
+
+This tool should be used when it's desirable to create a copy of a file or directory without modifying the original.
+It's much more efficient than doing this by separately reading and then writing the file or directory's contents,
+so this tool should be preferred over that approach whenever copying is the goal.