Feature to create directly addressed Todos when mentioned in beginning

What does this MR do?

Adds a feature to create a 'directly addressed' Todo when @user (or multiple users) are mentioned in the beginning of a note.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24976 (closed)

lib/banzai/querying.rb

 module Banzai
   module Querying
     module_function
+
     # Searches a Nokogiri document using a CSS query, optionally optimizing it
     # whenever possible.
     #
@@ -8,12 +9,12 @@ module Querying
     # query             - The CSS query to use.
     # reference_options - A hash with nodes filter options
     #
-    # Returns a Nokogiri::XML::NodeSet.
+    # Returns an array of Nokogiri::XML::Element objects if location is specified
+    # in reference_options. Otherwise it would a Nokogiri::XML::NodeSet.
     def css(document, query, reference_options = {})
       # When using "a.foo" Nokogiri compiles this to "//a[...]" but
       # "descendant::a[...]" is quite a bit faster and achieves the same result.
       xpath = Nokogiri::CSS.xpath_for(query)[0].gsub(%r{^//}, 'descendant::')
-
       nodes = document.xpath(xpath)
       filter_nodes(nodes, reference_options)
     end
@@ -26,25 +27,32 @@ def filter_nodes(nodes, reference_options)
       end
     end
 
-    # Selects nodes if they are present in the beginning of the document.
+    # Selects child nodes if they are present in the beginning among other siblings.
     #
     # nodes - A Nokogiri::XML::NodeSet.
     #
     # Returns an array of Nokogiri::XML::Element objects.
     def filter_nodes_at_beginning(nodes)
-      parent         = nodes.first.parent
-      children       = parent.children
-      nodes          = nodes.to_a
+      parents_and_nodes = nodes.group_by(&:parent)
       filtered_nodes = []
 
-      children.each do |child|
-        next if child.text.blank?
-        node = nodes.shift
-        break unless node == child
-        filtered_nodes << node
+      parents_and_nodes.each do |parent, nodes|
+        children = parent.children
+        nodes    = nodes.to_a
+
+        children.each do |child|
+          next if can_be_skipped?(child)
+          node = nodes.shift
+          break unless node == child
+          filtered_nodes << node
+        end
       end
 
       filtered_nodes
     end
+
+    def can_be_skipped?(node)
+      node.text.blank? || node.text.strip == ','
+    end
   end
 end