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/main_test.go

@@ -0,0 +1,27 @@
+// 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"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m, &unittest.TestOptions{
+		FixtureFiles: []string{"repository.yml", "user.yml"},
+		SetUp: func() error {
+			markup.RenderBehaviorForTesting.DisableAdditionalAttributes = true
+			markup.Init(&markup.RenderHelperFuncs{
+				IsUsernameMentionable: func(ctx context.Context, username string) bool {
+					return username == "user2"
+				},
+			})
+			return nil
+		},
+	})
+}