feat(helm): add RenderChart functions for template rendering

pkg/helm/fsloader.go

@@ -21,6 +21,8 @@ import (
 
 	"helm.sh/helm/v3/pkg/chart"
 	chartLoader "helm.sh/helm/v3/pkg/chart/loader"
+	"helm.sh/helm/v3/pkg/chartutil"
+	"helm.sh/helm/v3/pkg/engine"
 )
 
 // LoadChart loads a Helm chart from an fs.FS at the specified path.
@@ -72,3 +74,40 @@ func LoadChart(resourceFS fs.FS, chartPath string) (*chart.Chart, error) {
 
 	return loadedChart, nil
 }
+
+// RenderChart renders a Helm chart's templates with the provided values.
+// This does not require cluster access - it's a pure template rendering operation.
+// Returns a map of template name to rendered content.
+func RenderChart(resourceFS fs.FS, chartPath string, values Values, namespace, releaseName string) (map[string]string, error) {
+	loadedChart, err := LoadChart(resourceFS, chartPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to load chart: %w", err)
+	}
+
+	return RenderLoadedChart(loadedChart, values, namespace, releaseName)
+}
+
+// RenderLoadedChart renders an already-loaded chart's templates with the provided values.
+// Returns a map of template name to rendered content.
+func RenderLoadedChart(loadedChart *chart.Chart, values Values, namespace, releaseName string) (map[string]string, error) {
+	// Create release options for rendering
+	options := chartutil.ReleaseOptions{
+		Name:      releaseName,
+		Namespace: namespace,
+		IsInstall: true,
+	}
+
+	// Merge values with chart defaults
+	chartValues, err := chartutil.ToRenderValues(loadedChart, values, options, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create render values: %w", err)
+	}
+
+	// Render templates
+	rendered, err := engine.Render(loadedChart, chartValues)
+	if err != nil {
+		return nil, fmt.Errorf("failed to render chart templates: %w", err)
+	}
+
+	return rendered, nil
+}

pkg/helm/fsloader_test.go

@@ -15,11 +15,43 @@
 package helm
 
 import (
+	"maps"
 	"os"
+	"strings"
 	"testing"
 	"testing/fstest"
 )
 
+func TestRenderChart(t *testing.T) {
+	testFS := os.DirFS("testdata")
+
+	t.Run("renders template with values", func(t *testing.T) {
+		rendered, err := RenderChart(testFS, "chart", Values{"value": "hello"}, "test-ns", "my-release")
+		if err != nil {
+			t.Fatalf("expected no error, got: %v", err)
+		}
+
+		cm, ok := rendered["test-chart/templates/configmap.yaml"]
+		if !ok {
+			t.Fatalf("expected configmap template in output, got keys: %v", maps.Keys(rendered))
+		}
+
+		if !strings.Contains(cm, "namespace: test-ns") {
+			t.Errorf("expected namespace 'test-ns' in rendered output, got:\n%s", cm)
+		}
+		if !strings.Contains(cm, `value: "hello"`) {
+			t.Errorf("expected value 'hello' in rendered output, got:\n%s", cm)
+		}
+	})
+
+	t.Run("returns error for missing chart", func(t *testing.T) {
+		_, err := RenderChart(testFS, "nonexistent", nil, "ns", "rel")
+		if err == nil {
+			t.Fatal("expected error for non-existent chart path")
+		}
+	})
+}
+
 func TestLoadChart(t *testing.T) {
 	testFS := os.DirFS("testdata")