fix(statistics cookie): count only TODO headlines (#938)

Previously, the todo statistics cookie was counting all child headlines in
its calculation, regardless of whether they had TODO keywords. This fix
modifies the `update_todo_cookie` function to:

1. Filter child headlines to only those with valid TODO keywords
2. Calculate the ratio of DONE tasks to total tasks with TODO keywords
3. Update the cookie with the correct statistics

This ensures consistency with Org Mode's behavior where non-task headlines
are excluded from statistics calculations.

Co-authored-by: Sebastian Flügge <sebastian.fluegge@dnv.com>

Author: seflue

lua/orgmode/files/headline.lua

@@ -946,14 +946,19 @@ function Headline:update_todo_cookie()
     return self
   end
 
-  -- Count done children headlines
+  -- Count done children headlines and total children with TODO keywords
   local children = self:get_child_headlines()
+  local headlines_with_todo = vim.tbl_filter(function(h)
+    local todo, _, _ = h:get_todo()
+    return todo ~= nil
+  end, children)
+
   local dones = vim.tbl_filter(function(h)
     return h:is_done()
-  end, children)
+  end, headlines_with_todo)
 
   -- Set the cookie
-  return self:_set_cookie(cookie, #dones, #children)
+  return self:_set_cookie(cookie, #dones, #headlines_with_todo)
 end
 
 function Headline:update_parent_cookie()

tests/plenary/ui/mappings/todo_spec.lua

@@ -430,4 +430,25 @@ describe('Todo mappings', function()
       '** TODO item',
     }, vim.api.nvim_buf_get_lines(0, 0, 6, false))
   end)
+  it('should update headline cookies when children todo state changes', function()
+    helpers.create_file({
+      '* Test orgmode [/]',
+      '** TODO item',
+      '** TODO item',
+      '** TODO item',
+      '** Non-todo item',
+    })
+    vim.fn.cursor(4, 1)
+    local now = Date.now()
+    -- Changing to DONE and adding closed date
+    vim.cmd([[norm citd]])
+    assert.are.same({
+      '* Test orgmode [1/3]',
+      '** TODO item',
+      '** TODO item',
+      '** DONE item',
+      '   CLOSED: [' .. now:to_string() .. ']',
+      '** Non-todo item',
+    }, vim.api.nvim_buf_get_lines(0, 0, 6, false))
+  end)
 end)