fix(diagnostics)!: restore is_pull namespace argument #38698

Problem:
The current LSP diagnostic implementation can't differ between a pull
diagnostic with no identifier and a set of diagnostics provided via push
diagnostics.

"Anonymous pull providers" are expected by the protocol https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticOptions
, depending on how the capability was registered:
- Dynamic registrations have an identifier.
- Static registrations will not.

Solution:
Restore the `is_pull` argument removed in
https://github.com/neovim/neovim/pull/37938, keeping the identifier of
pull diagnostic collections.

runtime/lua/vim/lsp/buf.lua

@@ -1358,7 +1358,7 @@ function M.code_action(opts)
       local lnum = api.nvim_win_get_cursor(0)[1] - 1
 
       client:_provider_foreach('textDocument/diagnostic', function(cap)
-        local ns_pull = lsp.diagnostic.get_namespace(client.id, cap.identifier)
+        local ns_pull = lsp.diagnostic.get_namespace(client.id, true, cap.identifier)
         vim.list_extend(
           diagnostics,
           vim.diagnostic.get(bufnr, { namespace = ns_pull, lnum = lnum })