fix: use table.concat instead of vim.fn.join for provider list

The assert was joining providers with `vim.fn.join`, which calls into
Vimscript and can fail in fast event contexts (e.g. during async
completion).

Replaces it with Lua's built-in `table.concat`, removing the unnecessary
Vimscript bridge call and thus potential errors.

Author: soifou

lua/blink/cmp/sources/lib/init.lua

@@ -125,7 +125,7 @@ function sources.get_provider_by_id(provider_id)
     'Requested provider "'
       .. provider_id
       .. '" has not been configured. Available providers: '
-      .. vim.fn.join(vim.tbl_keys(sources.providers), ', ')
+      .. table.concat(vim.tbl_keys(sources.providers), ', ')
   )
 
   -- initialize the provider if it hasn't been initialized yet