assistant2: Use snake_case for tool names (#27903)

The [Gemini
docs](https://ai.google.dev/gemini-api/docs/function-calling?example=weather#function_declarations)
state that function names should be in `snake_case` or `camelCase`.

Release Notes:

- N/A

crates/assistant_tools/src/read_file_tool.rs

@@ -44,7 +44,7 @@ pub struct ReadFileTool;
 
 impl Tool for ReadFileTool {
     fn name(&self) -> String {
-        "read-file".into()
+        "read_file".into()
     }
 
     fn needs_confirmation(&self) -> bool {