Fix markup heading parsing, fix emphasis parsing (#36284)

Fixes #36106, fix #17958

---------

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

modules/markup/html_toc_test.go

@@ -0,0 +1,60 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup_test
+
+import (
+	"regexp"
+	"testing"
+
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/markup/markdown"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestToCWithHTML(t *testing.T) {
+	defer test.MockVariableValue(&markup.RenderBehaviorForTesting.DisableAdditionalAttributes, true)()
+
+	t1 := `tag <a href="link">link</a> and <b>Bold</b>`
+	t2 := "code block `<a>`"
+	t3 := "markdown **bold**"
+	input := `---
+include_toc: true
+---
+
+# ` + t1 + `
+## ` + t2 + `
+#### ` + t3 + `
+## last
+`
+
+	renderCtx := markup.NewTestRenderContext().WithEnableHeadingIDGeneration(true)
+	resultHTML, err := markdown.RenderString(renderCtx, input)
+	assert.NoError(t, err)
+	result := string(resultHTML)
+	re := regexp.MustCompile(`(?s)<details class="frontmatter-content">.*?</details>`)
+	result = re.ReplaceAllString(result, "\n")
+	expected := `<details><summary>toc</summary>
+<ul>
+  <li><a href="#user-content-tag-link-and-bold" rel="nofollow">tag link and Bold</a></li>
+  <ul>
+    <li><a href="#user-content-code-block-a" rel="nofollow">code block &lt;a&gt;</a></li>
+    <ul>
+      <ul>
+        <li><a href="#user-content-markdown-bold" rel="nofollow">markdown bold</a></li>
+      </ul>
+    </ul>
+    <li><a href="#user-content-last" rel="nofollow">last</a></li>
+  </ul>
+</ul>
+</details>
+
+<h1 id="user-content-tag-link-and-bold">tag <a href="/link" rel="nofollow">link</a> and <b>Bold</b></h1>
+<h2 id="user-content-code-block-a">code block <code>&lt;a&gt;</code></h2>
+<h4 id="user-content-markdown-bold">markdown <strong>bold</strong></h4>
+<h2 id="user-content-last">last</h2>
+`
+	assert.Equal(t, expected, result)
+}