fix(lsp): _cancel_all_requests() tries to cancel completed requests #34105

Problem:
The cancel function returned by `vim.lsp.buf_request` tries to cancel
all the requests, including those that have already been completed,
causing "Cannot find request with id ... whilst attempting to cancel"
errors to be logged when it is called.

Solution:
Only cancel the requests that are present in `client.requests`.

(cherry picked from commit a9b8a8dc6c)

runtime/lua/vim/lsp.lua

@@ -1288,7 +1288,9 @@ function lsp.buf_request(bufnr, method, params, handler, on_unsupported)
   local function _cancel_all_requests()
     for client_id, request_id in pairs(client_request_ids) do
       local client = all_clients[client_id]
-      client:cancel_request(request_id)
+      if client.requests[request_id] then
+        client:cancel_request(request_id)
+      end
     end
   end