Refactor markup render system (#32612)

This PR removes (almost) all path tricks, and introduces "renderhelper"
package.

Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.

Fix #31411 , fix #18592, fix #25632 and maybe more problems. (ps: fix
#32608 by the way)

models/renderhelper/simple_document_test.go

@@ -0,0 +1,40 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package renderhelper
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/markup/markdown"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSimpleDocument(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+	rctx := NewRenderContextSimpleDocument(context.Background(), "/base").WithMarkupType(markdown.MarkupName)
+	rendered, err := markup.RenderString(rctx, `
+65f1bf27bc3bf70f64657658635e66094edbcb4d
+#1
+@user2
+
+[/test](/test)
+[./test](./test)
+![/image](/image)
+![./image](./image)
+`)
+	assert.NoError(t, err)
+	assert.Equal(t,
+		`<p>65f1bf27bc3bf70f64657658635e66094edbcb4d
+#1
+<a href="/base/user2" rel="nofollow">@user2</a></p>
+<p><a href="/base/test" rel="nofollow">/test</a>
+<a href="/base/test" rel="nofollow">./test</a>
+<a href="/base/image" target="_blank" rel="nofollow noopener"><img src="/base/image" alt="/image"/></a>
+<a href="/base/image" target="_blank" rel="nofollow noopener"><img src="/base/image" alt="./image"/></a></p>
+`, rendered)
+}