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