fix(engine): make toToml return empty string on error, add mustToToml

Previously, toToml returned err.Error() on marshal failure, which was
inconsistent with toYaml and toJson (both return "" on error).

This commit:
- Changes toToml to return "" on error, aligning it with toYaml/toJson
- Adds mustToToml that panics on error, consistent with mustToYaml/mustToJson
- Registers mustToToml in the template function map
- Adds tests covering the error-swallowing behavior and mustToToml

Fixes #31430

Signed-off-by: Ilya Kiselev <kis-ilya-a@yandex.ru>

pkg/engine/funcs.go

@@ -50,6 +50,7 @@ func funcMap() template.FuncMap {
 	// Add some extra functionality
 	extra := template.FuncMap{
 		"toToml":        toTOML,
+		"mustToToml":    mustToTOML,
 		"fromToml":      fromTOML,
 		"toYaml":        toYAML,
 		"mustToYaml":    mustToYAML,
@@ -157,7 +158,23 @@ func toTOML(v any) string {
 	e := toml.NewEncoder(b)
 	err := e.Encode(v)
 	if err != nil {
-		return err.Error()
+		// Swallow errors inside of a template.
+		return ""
+	}
+	return b.String()
+}
+
+// mustToTOML takes an interface, marshals it to toml, and returns a string.
+// It will panic if there is an error.
+//
+// This is designed to be called from a template when need to ensure that the
+// output TOML is valid.
+func mustToTOML(v any) string {
+	b := bytes.NewBuffer(nil)
+	e := toml.NewEncoder(b)
+	err := e.Encode(v)
+	if err != nil {
+		panic(err)
 	}
 	return b.String()
 }

pkg/engine/funcs_test.go

@@ -86,6 +86,11 @@ keyInElement1 = "valueInElement1"`,
 		tpl:    `{{ toToml . }}`,
 		expect: "[mast]\n  sail = \"white\"\n",
 		vars:   map[string]map[string]string{"mast": {"sail": "white"}},
+	}, {
+		// toToml should return empty string on error, not the error message
+		tpl:    `{{ toToml . }}`,
+		expect: "", // should return empty string and swallow error
+		vars:   map[int]string{1: "one"},
 	}, {
 		tpl:    `{{ fromYaml . }}`,
 		expect: "map[Error:error unmarshaling JSON: while decoding JSON: json: cannot unmarshal array into Go value of type map[string]interface {}]",
@@ -159,6 +164,13 @@ keyInElement1 = "valueInElement1"`,
 		tpl:    `{{ toJson . }}`,
 		expect: "", // should return empty string and swallow error
 		vars:   loopMap,
+	}, {
+		tpl:  `{{ mustToToml . }}`,
+		vars: map[int]string{1: "one"}, // non-string key is invalid in TOML
+	}, {
+		tpl:    `{{ mustToToml . }}`,
+		expect: "foo = \"bar\"\n", // should succeed and return TOML string
+		vars:   map[string]string{"foo": "bar"},
 	},
 	}