test(tools): cover missing field constructor

Harvests focused coverage from #3133.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Co-authored-by: Hmbown <101357273+Hmbown@users.noreply.github.com>

crates/tools/src/lib.rs

@@ -560,6 +560,12 @@ mod tests {
         );
     }
 
+    #[test]
+    fn tool_error_missing_field_constructor() {
+        let err = ToolError::missing_field("my_field");
+        assert!(matches!(err, ToolError::MissingField { field } if field == "my_field"));
+    }
+
     #[test]
     fn tool_error_invalid_input_creates_correct_variant() {
         let err = ToolError::invalid_input("test invalid message");