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`:

Beginne mit der Überprüfung von packages/devtools_app/pubspec.yaml und den Icon-Referenzen in packages/devtools_app/lib/src/screens/debugger/controls.dart und untersuche anschließend die betroffenen golden files unter packages/devtools_app/test/test_infra/goldens. Führe die macOS golden tests aus und vergleiche die Darstellung der vier aufgeführten Assets; abgeschlossen ist die Aufgabe, wenn die erwarteten Icons in den betroffenen Goldens konsistent erscheinen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.