`dart fix --dry-run` should support returning non-zero code on changes for CI uses

[artoale]

This is equivalent to #44023 , but for dart fix
We just assumed it would return non zero when there are fixes to apply in our CI, and later realised it does not :)

• Dart 3.5.1 (stable) (Tue Aug 13 21:02:17 2024 +0000) on "macos_arm64"
• on macos / Version 14.6.1 (Build 23G93)
• locale is en-IT

[dart-github-bot]

Summary: The dart fix --dry-run command currently does not return a non-zero exit code when fixes are available, making it difficult to use in CI environments. This issue proposes adding this functionality for consistency with similar tools.