fix(lsp): on detach, cancel pending foldingRange requests #31509

Problem:
1. Open a relatively large file (so the server needs some time to
   process the request).
2. Then immediately execute `:bdelete`.
3. Once the request is completed, the handler will obtain the bufstate
   of a buffer already unloaded.

    Error executing vim.schedule lua callback: ...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:119: assertion failed!
    stack traceback:
            [C]: in function 'assert'
            ...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:119: in function 'multi_handler'
            ...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:140: in function 'handler'
            ...HEAD-c137841_1/share/nvim/runtime/lua/vim/lsp/client.lua:669: in function ''
            vim/_editor.lua: in function <vim/_editor.lua:0>

Solution:
On detach, cancel all pending textDocument_foldingRange requests.

runtime/lua/vim/lsp/_folding_range.lua

@@ -197,6 +197,11 @@ local function setup(bufnr)
     -- `on_detach` also runs on buffer reload (`:e`).
     -- Ensure `bufstate` and hooks are cleared to avoid duplication or leftover states.
     on_detach = function()
+      util._cancel_requests({
+        bufnr = bufnr,
+        method = ms.textDocument_foldingRange,
+        type = 'pending',
+      })
       bufstates[bufnr] = nil
       api.nvim_clear_autocmds({ buffer = bufnr, group = augroup_setup })
     end,