fix(treesitter): clamp unbounded fold ranges

Problem:
Tree-sitter uses UINT32_MAX for full-document ranges, which becomes -1 on 32-bit platforms and reaches _foldupdate as an invalid end row.

Solution:
Treat negative changed-range end rows as unbounded and clamp them to the buffer line count. Add a regression test that simulates the 32-bit sentinel.

AI-assisted: Codex

runtime/lua/vim/treesitter/_fold.lua

@@ -291,9 +291,10 @@ local function on_changedtree(bufnr, tree_changes)
       local srow, _, erow, ecol = Range.unpack4(change)
       -- If a parser doesn't have any ranges explicitly set, treesitter will
       -- return a range with end_row and end_bytes with a value of UINT32_MAX,
-      -- so clip end_row to the max buffer line.
+      -- which is represented as -1 on 32-bit platforms, so clip end_row to
+      -- the max buffer line.
       -- TODO(lewis6991): Handle this generally
-      if erow > max_erow then
+      if erow > max_erow or erow < 0 then
         erow = max_erow
       elseif ecol > 0 then
         erow = erow + 1

test/functional/treesitter/fold_spec.lua

@@ -824,4 +824,42 @@ t2]])
                                               |
     ]])
   end)
+
+  it('clamps unbounded changed ranges on 32-bit platforms', function()
+    insert([[
+one
+two]])
+
+    exec_lua(function()
+      local parser = {}
+
+      function parser:parse(_, callback)
+        if callback then
+          callback(nil, {})
+        end
+      end
+
+      function parser:for_each_tree() end
+
+      function parser:register_cbs(callbacks)
+        self.callbacks = callbacks
+      end
+
+      vim.treesitter.get_parser = function()
+        return parser
+      end
+
+      vim.treesitter.foldexpr()
+      vim.wo.foldmethod = 'expr'
+      vim._foldupdate = function(_, first, last)
+        _G.foldupdate_range = { first, last }
+      end
+
+      -- UINT32_MAX is represented as -1 by Lua integers on 32-bit platforms.
+      parser.callbacks.on_changedtree({ { 0, 0, -1, -1 } })
+    end)
+    poke_eventloop()
+
+    eq({ 0, 2 }, exec_lua('return _G.foldupdate_range'))
+  end)
 end)