feat(lsp): attach to buftype=help buffers #38412

Problem: `vim.lsp.enable()` skips all buffers with non-empty `buftype`,
including `help` buffers. LSPs targeting `filetype='help'` never
auto-attach despite help buffers being real files.

Solution: Expand the `buftype` guard in `lsp_enable_callback` to allow
`help` alongside `""`.

runtime/lua/vim/lsp.lua

@@ -520,8 +520,8 @@ end
 
 --- @param bufnr integer
 local function lsp_enable_callback(bufnr)
-  -- Only ever attach to buffers that represent an actual file.
-  if vim.bo[bufnr].buftype ~= '' then
+  -- Only ever attach to buffers (including "help") that represent an actual file.
+  if vim.bo[bufnr].buftype ~= '' and vim.bo[bufnr].buftype ~= 'help' then
     return
   end