refactor: external render (#38885) (#38898)

Backport #38885 by @wxiaoguang

make the "command variable replacement" more accurate and
OS-independent, add a test for it.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/external/external_test.go

@@ -0,0 +1,24 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package external
+
+import (
+	"testing"
+
+	"gitea.dev/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPrepareExternalCommand(t *testing.T) {
+	r := &Renderer{MarkupRenderer: &setting.MarkupRenderer{Command: ""}}
+	_, _, err := r.prepareExternalCommand(map[string]string{"KEY": "val"})
+	assert.ErrorContains(t, err, "no command")
+
+	r = &Renderer{MarkupRenderer: &setting.MarkupRenderer{Command: `"/foo bar/bin" --opt $KEY "$KEY" %KEY% other`}}
+	prog, args, err := r.prepareExternalCommand(map[string]string{"KEY": `a"b`})
+	assert.NoError(t, err)
+	assert.Equal(t, "/foo bar/bin", prog)
+	assert.Equal(t, []string{"--opt", `a"b`, `a"b`, `a"b`, "other"}, args)
+}