fix(ruby): run ruby-lsp on the project's Ruby via rbenv

Mason's ruby-lsp is a gem bound to whichever Ruby installed it. In a
project that pins a different Ruby, loading the project bundle fails
with Bundler::RubyVersionMismatch and the server exits, which is what
kept producing the client-quit warning. ruby-lsp's editor docs advise
against Mason for this reason and recommend the version manager's
shim.

Turn Mason off for ruby_lsp and launch ~/.rbenv/shims/ruby-lsp from
the project root, since rbenv resolves .ruby-version from the working
directory and Neovim otherwise starts servers in its own cwd. Machines
without rbenv fall back to ruby-lsp on PATH.

lua/plugins/ruby.lua

@@ -0,0 +1,22 @@
+return {
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        ruby_lsp = {
+          -- ruby-lsp must run on the project's Ruby to load its bundle, so use
+          -- the rbenv shim rather than Mason's copy, which is tied to whichever
+          -- Ruby installed it.
+          mason = false,
+          cmd = function(dispatchers, config)
+            local shim = vim.fn.expand("~/.rbenv/shims/ruby-lsp")
+            local exe = vim.fn.executable(shim) == 1 and shim or "ruby-lsp"
+            -- rbenv reads .ruby-version from the working directory, and nvim
+            -- otherwise starts servers in its own cwd.
+            return vim.lsp.rpc.start({ exe }, dispatchers, { cwd = config.root_dir })
+          end,
+        },
+      },
+    },
+  },
+}