Missing icon assets in golden tests

Filing this to investigate why some of the assets we use as icon images are missing during macos golden testing.

The changes in https://github.com/flutter/devtools/pull/9304/files caused some icons that weren't being rendered previously in our macos golden tests to appear, as well as others that were being rendered in our macos golden tests to disappear.

The assets that were previously missing are:
* `packages/devtools_app/icons/material_symbols/step_over.png`
* `packages/devtools_app/icons/material_symbols/step_into.png`
* `packages/devtools_app/icons/material_symbols/step_out.png`

The asset that is now missing is:

* `packages/devtools_app/icons/flutter.png`

All 4 of the assets display as expected in a running app, and all 4 are included in the pubspec:
* https://github.com/flutter/devtools/blob/8e6559236a6464104e05ced03be1a2ca80e7f725/packages/devtools_app/pubspec.yaml#L96
* https://github.com/flutter/devtools/blob/8e6559236a6464104e05ced03be1a2ca80e7f725/packages/devtools_app/pubspec.yaml#L100

---

### `packages/devtools_app/test/test_infra/goldens/codeview_scrollbars.png`:

Note: The code for these buttons was not touched in https://github.com/flutter/devtools/pull/9304: https://github.com/flutter/devtools/blob/a62c33884a355eef0bd351c8e0d5778d393c0b0b/packages/devtools_app/lib/src/screens/debugger/controls.dart#L121-L145

### `packages/devtools_app/test/test_infra/goldens/logging/metadata_chips.png`

Note: The code for these icons also didn't change, except for the icon size becoming `const`:

首先检查 packages/devtools_app/pubspec.yaml 以及 packages/devtools_app/lib/src/screens/debugger/controls.dart 中的图标引用，然后检查 packages/devtools_app/test/test_infra/goldens 下受影响的 golden files。运行 macOS golden tests，并比较列出的四个 asset 的渲染结果；当预期图标在受影响的 goldens 中一致显示时，即表示完成。

由索引模型根据 Issue 内容生成。