feat(tmpl): fails when key does not exist (#916)

The templates used in Feature SDK are not intended (at least right now) to provide conditional rendering like in Helm. Thus we should fail fast if the Feature's underlying map does not provide the key referred to in a template. 

This is achieved by passing the parsing option (missingkey=error) to built-in Golang templating.

> [!NOTE]
> This works out of the box with `struct`s but we should also fail when dealing with `map`s.

pkg/feature/manifest.go

@@ -88,7 +88,10 @@ func (t *templateManifest) Process(data any) ([]*unstructured.Unstructured, erro
 		return nil, fmt.Errorf("failed to create file: %w", err)
 	}
 
-	tmpl, err := template.New(t.name).Funcs(template.FuncMap{"ReplaceChar": ReplaceChar}).Parse(string(content))
+	tmpl, err := template.New(t.name).
+		Option("missingkey=error").
+		Funcs(template.FuncMap{"ReplaceChar": ReplaceChar}).
+		Parse(string(content))
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse template: %w", err)
 	}

pkg/feature/manifest_test.go

@@ -75,8 +75,7 @@ data:
 	})
 
 	Describe("Templated Manifest Processing", func() {
-		BeforeEach(func() {
-			resourceYaml := `
+		resourceYaml := `
 apiVersion: v1
 kind: ConfigMap
 metadata:
@@ -85,11 +84,29 @@ metadata:
 data:
   key: Data
 `
+
+		BeforeEach(func() {
 			path = "path/to/template.yaml"
 			err := afero.WriteFile(inMemFS.Afs, path, []byte(resourceYaml), 0644)
 			Expect(err).ToNot(HaveOccurred())
 		})
 
+		It("should fail when template refers to non existing key", func() {
+			// given
+			pathToBrokenTpl := filepath.Join("broken", path)
+			Expect(afero.WriteFile(inMemFS.Afs, pathToBrokenTpl, []byte(resourceYaml+"\n {{ .NotExistingKey }}"), 0644)).To(Succeed())
+			data := map[string]string{
+				"TargetNamespace": "template-ns",
+			}
+			manifest := feature.CreateTemplateManifestFrom(inMemFS, pathToBrokenTpl)
+
+			// when
+			_, err := manifest.Process(data)
+
+			// then
+			Expect(err).Should(MatchError(ContainSubstring("at <.NotExistingKey>: map has no entry for key")))
+		})
+
 		It("should substitute target namespace in the templated manifest", func() {
 			// given
 			data := feature.Spec{