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/assistant_tools/src/read_file_tool.rs

@@ -68,7 +68,7 @@ impl Tool for ReadFileTool {
     }
 
     fn icon(&self) -> IconName {
-        IconName::ToolSearch
+        IconName::ToolRead
     }
 
     fn input_schema(&self, format: LanguageModelToolSchemaFormat) -> Result<serde_json::Value> {