fix(lsp): improve diagnostics handling and comments

- Add TODO comments for aggregating diagnostics from all pull namespaces
  and for clearing diagnostics when an empty array is received, referencing
  the LSP specification.
- Update diagnostics refresh logic to safely access previousResultId,
  preventing potential nil errors.

runtime/lua/vim/lsp/buf.lua

@@ -1365,6 +1365,7 @@ function M.code_action(opts)
       params.context = context
     else
       local ns_push = lsp.diagnostic.get_namespace(client.id, false)
+      -- TODO(tris203): should we aggregate diagnostics from all the possible pull namespaces?
       local ns_pull = lsp.diagnostic.get_namespace(client.id, true)
       local diagnostics = {}
       local lnum = api.nvim_win_get_cursor(0)[1] - 1

runtime/lua/vim/lsp/diagnostic.lua

@@ -261,6 +261,8 @@ end
 ---@param params lsp.PublishDiagnosticsParams
 ---@param ctx lsp.HandlerContext
 function M.on_publish_diagnostics(_, params, ctx)
+  -- TODO(tris203): if empty array then clear diags
+  -- https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_publishDiagnostics
   handle_diagnostics(params.uri, ctx.client_id, params.diagnostics, false)
 end
 
@@ -396,7 +398,9 @@ function M._refresh(bufnr, client_id, only_visible)
       local params = {
         identifier = cap.identifier,
         textDocument = util.make_text_document_params(bufnr),
-        previousResultId = bufstate.client_result_id[key],
+        previousResultId = bufstate
+          and bufstate.client_result_id
+          and bufstate.client_result_id[key],
       }
       client:request(method, params, nil, bufnr)
     end)