[feat req] Artifact exists sub action #459

# Query GitHub REST API to check if an artifact with the given name exists
  # This only works for artifacts uploaded with actions/upload-artifact@v4
  - name: artifact exists
    uses: actions/github-script@v8
    id: artifact-exists
    with:
      script: |
        const artifactName = "${{inputs.artifact_name}}";
        
        // If artifact name is empty, skip the check (allows disabling artifact lookup)
        if (!artifactName) {
            console.log("Artifact name is empty. Not looking for any github artifact.");
            core.setOutput("exists", false);
            return;
        }
        
        try {
            // Call GitHub REST API to list all artifacts from this workflow run
            const response = await github.rest.actions.listWorkflowRunArtifacts({
              owner: context.repo.owner,
              repo: context.repo.repo,
              run_id: context.runId
            });
            console.log(response.data)
            
            const artifacts = response.data.artifacts;
            
            // Search for artifact matching the name, and verify it's valid:
            // - Not expired (still accessible)
            // - Has content (size > 0 bytes)
            const artifactExists =  artifacts.find(a =>
                a.name === artifactName &&
                !a.expired &&
                a.size_in_bytes > 0
            );
            
            // Output boolean indicating whether a valid artifact was found
            core.setOutput("exists", Boolean(artifactExists));
        } catch (e) {
          // Handle errors gracefully - don't fail the workflow
          console.error("Error listing artifacts", e);
          core.setOutput("exists", false);
        }