[v10.0/forgejo] fix: render link in heading correctly in TOC (#6859)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/6853

- When you use a link in a heading such as `# [Text](link)` (instead of the conventional `# Text`) the TOC should only show `Text` and not `[Text](link)`.
- Use the `mdutil.Text` to only get the text from actual text nodes and not the text that was provided in the markdown input.
- Regression of e2fddcf681
- Resolves forgejo/forgejo#6847
- Added integration test.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6859
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>

modules/markup/markdown/transform_heading.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 
 	"code.gitea.io/gitea/modules/markup"
+	mdutil "code.gitea.io/gitea/modules/markup/markdown/util"
 	"code.gitea.io/gitea/modules/util"
 
 	"github.com/yuin/goldmark/ast"
@@ -19,7 +20,7 @@ func (g *ASTTransformer) transformHeading(_ *markup.RenderContext, v *ast.Headin
 			v.SetAttribute(attr.Name, []byte(fmt.Sprintf("%v", attr.Value)))
 		}
 	}
-	txt := v.Lines().Value(reader.Source())
+	txt := mdutil.Text(v, reader.Source())
 	header := markup.Header{
 		Text:  util.UnsafeBytesToString(txt),
 		Level: v.Level,