fix(ruby_lsp): prevent duplicate clients on second buffer

Problem: When opening multiple Ruby buffers, a second LSP client is
spawned instead of reusing the first client, even though they share
the same root directory.

Root cause: The reuse_client function sets cmd_cwd as a side effect
during reuse checks. This means the first client is created without
cmd_cwd set, causing the reuse check to fail for the second buffer
(nil != root_dir). The second client gets cmd_cwd set (due to the
side effect), so subsequent buffers correctly reuse it.

Solution: Use on_new_config to ensure cmd_cwd is always set before
client creation. Update reuse_client to compare without mutation,
with fallback to root_dir for backward compatibility.

Author: stask

lsp/ruby_lsp.lua

@@ -20,8 +20,12 @@ return {
   init_options = {
     formatter = 'auto',
   },
+  on_new_config = function(config, root_dir)
+    config.cmd_cwd = root_dir
+  end,
   reuse_client = function(client, config)
-    config.cmd_cwd = config.root_dir
-    return client.config.cmd_cwd == config.cmd_cwd
+    local client_cwd = client.config.cmd_cwd or client.root_dir
+    local config_cwd = config.cmd_cwd or config.root_dir
+    return client_cwd == config_cwd
   end,
 }