agent: Make all icons used for tool calls consistent (#34160)

Starting to use the `Tool...` family of icons dedicated & designed for
the agent panel.

Release Notes:

- N/A

crates/assistant_tools/src/thinking_tool.rs

@@ -37,7 +37,7 @@ impl Tool for ThinkingTool {
     }
 
     fn icon(&self) -> IconName {
-        IconName::LightBulb
+        IconName::ToolBulb
     }
 
     fn input_schema(&self, format: LanguageModelToolSchemaFormat) -> Result<serde_json::Value> {