agent: Fix edit bar's background color when zoomed in (#34163)

When the panel was zoomed in, the edit bar's background color would have
a different color than the rest of the panel. This PR fixes it by using
the `panel_background` color token.

Release Notes:

- N/A

crates/icons/src/icons.rs

@@ -258,6 +258,7 @@ pub enum IconName {
     ToolHammer,
     ToolNotification,
     ToolPencil,
+    ToolRead,
     ToolRegex,
     ToolSearch,
     ToolTerminal,