Fix task list checkbox toggle to work with YAML front matter (#25184) (#25236)

Backport https://github.com/go-gitea/gitea/pull/25184 by @jtran
Closes #25225.

Fixes https://github.com/go-gitea/gitea/issues/25160.

`data-source-position` of checkboxes in a task list was incorrect
whenever there was YAML front matter. This would result in issue content
or PR descriptions getting corrupted with random `x` or space characters
when a user checked or unchecked a task.

(cherry picked from commit 1650a26eb5)

modules/markup/markdown/markdown.go

@@ -173,6 +173,9 @@ func actualRender(ctx *markup.RenderContext, input io.Reader, output io.Writer)
 	}
 	buf = giteautil.NormalizeEOL(buf)
 
+	// Preserve original length.
+	bufWithMetadataLength := len(buf)
+
 	rc := &RenderConfig{
 		Meta: "table",
 		Icon: "table",
@@ -180,6 +183,12 @@ func actualRender(ctx *markup.RenderContext, input io.Reader, output io.Writer)
 	}
 	buf, _ = ExtractMetadataBytes(buf, rc)
 
+	metaLength := bufWithMetadataLength - len(buf)
+	if metaLength < 0 {
+		metaLength = 0
+	}
+	rc.metaLength = metaLength
+
 	pc.Set(renderConfigKey, rc)
 
 	if err := converter.Convert(buf, lw, parser.WithContext(pc)); err != nil {