fix: stream platform builder progress

platform/service/distribution_builder_test.go

@@ -185,6 +185,58 @@ func TestDockerDistributionBuilderKeepsSecretInIsolatedInput(t *testing.T) {
 	}
 }
 
+func TestDockerDistributionBuilderStreamsProgressMarkers(t *testing.T) {
+	sourceDir := createBuilderSource(t)
+	workspaceDir := t.TempDir()
+	var progress []DistributionBuildProgress
+	builder := NewDockerDistributionBuilder(DockerDistributionBuilderConfig{
+		DockerBinary: "docker-test",
+		Image:        "browser-platform-distribution-builder:1.0.0",
+		SourceDir:    sourceDir,
+		WorkspaceDir: workspaceDir,
+		CommandRunner: func(_ context.Context, _ string, args ...string) ([]byte, error) {
+			if len(args) > 0 && (args[0] == "version" || args[0] == "image") {
+				return []byte("27.0.0"), nil
+			}
+			return nil, errors.New("unexpected non-stream command")
+		},
+		CommandStream: func(_ context.Context, _ string, args []string, onLine func(string)) ([]byte, error) {
+			onLine("__platform_builder_progress__|72|build_compile: compiling run target executable")
+			outputDir := builderMountHostPath(t, args, "/workspace/output")
+			if err := os.WriteFile(filepath.Join(outputDir, "run"), []byte("compiled-run"), 0o700); err != nil {
+				return nil, err
+			}
+			return []byte("builder finished"), nil
+		},
+	})
+	payload, err := builder.BuildWithProgress(domain.DistributionBuildInput{
+		JobID:          "job-progress",
+		ComponentKind:  domain.DistributionComponentRun,
+		PluginID:       "game.scum",
+		TargetOS:       "linux",
+		TargetArch:     "amd64",
+		OutputFilename: "run",
+		AuthKey:        "component-key",
+	}, func(item DistributionBuildProgress) {
+		progress = append(progress, item)
+	})
+	if err != nil {
+		t.Fatalf("build with progress: %v", err)
+	}
+	if string(payload) != "compiled-run" {
+		t.Fatalf("unexpected built payload %q", payload)
+	}
+	found := false
+	for _, item := range progress {
+		if item.Percent == 72 && item.Message == "build_compile: compiling run target executable" {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatalf("streamed progress marker was not reported: %+v", progress)
+	}
+}
+
 func TestDockerDistributionBuilderRedactsFailureAndTimeout(t *testing.T) {
 	sourceDir := createBuilderSource(t)
 	workspaceDir := t.TempDir()