fix(lsp): do not detach from buffer if there are uninitialized clients (#29043)

Problem: if on_lines is called before the LSP is initialized, the buffer
is detached.
Solution: check for uninitialized clients before detaching.

(cherry picked from commit 292365fa1b)

Co-authored-by: Ilia Choly <ilia.choly@gmail.com>

runtime/lua/vim/lsp.lua

@@ -573,7 +573,8 @@ local function buf_attach(bufnr)
   api.nvim_buf_attach(bufnr, false, {
     on_lines = function(_, _, changedtick, firstline, lastline, new_lastline)
       if #lsp.get_clients({ bufnr = bufnr }) == 0 then
-        return true -- detach
+        -- detach if there are no clients
+        return #lsp.get_clients({ bufnr = bufnr, _uninitialized = true }) == 0
       end
       util.buf_versions[bufnr] = changedtick
       changetracking.send_changes(bufnr, firstline, lastline, new_lastline)