fix(lsp): retrigger diagnostics request on server cancellation (#31345)

Co-authored-by: Jesse <github@jessebakker.com>

runtime/lua/vim/lsp/handlers.lua

@@ -659,7 +659,8 @@ for k, fn in pairs(M) do
       })
     end
 
-    if err then
+    -- ServerCancelled errors should be propagated to the request handler
+    if err and err.code ~= protocol.ErrorCodes.ServerCancelled then
       -- LSP spec:
       -- interface ResponseError:
       --  code: integer;