Workflow fails to resolve Dart Code Metrics action in mobile-app-code-analysis

**What were you trying to achieve?**
Run the `mobile-app-code-analysis` GitHub Actions workflow for PR #3949 successfully.

**What are the expected results?**
The workflow should resolve the Dart Code Metrics action and proceed to metrics/analysis steps without failing during action resolution.

**What are the received results?**
The job fails immediately during workflow preparation with:

`Unable to resolve action dart-code-checker/dart-code-metrics-action, not found`

The failure occurs in `.github/workflows/dart-code-metrics.yaml` in the `metrics` job at the `Dart Code Metrics Action` step.

**What are the steps to reproduce the issue?**
1. Open or run PR #3949.
2. Trigger the `mobile-app-code-analysis` workflow.
3. Observe that the `metrics` job fails before execution because GitHub cannot resolve the action reference.

**In what environment did you encounter the issue?**
- OS: GitHub Actions runner
- Version: ubuntu-latest

**Additional context**
Workflow file: `.github/workflows/dart-code-metrics.yaml`

The current step uses:

```yaml
- name: Dart Code Metrics Action
uses: dart-code-checker/dart-code-metrics-action@v4.0.0
```

This action reference appears to be invalid or unavailable, causing the workflow to fail before code analysis starts.

Suggested fix: update the step to a valid, existing Dart Code Metrics action reference and pin it to a known-good release tag.