Specifically handle the case where bin/rails is not found

When the file is not found, it is likely that the LSP is not being run in
the root of a Rails project. Having a more specific error message will
help the user understand what is going on.

lib/ruby_lsp/ruby_lsp_rails/runner_client.rb

@@ -12,7 +12,15 @@ class << self
 
         sig { returns(RunnerClient) }
         def create_client
-          new
+          if File.exist?("bin/rails")
+            new
+          else
+            warn(<<~MSG)
+              Ruby LSP Rails failed to locate bin/rails in the current directory: #{Dir.pwd}"
+            MSG
+            warn("Server dependent features will not be available")
+            NullClient.new
+          end
         rescue Errno::ENOENT, StandardError => e # rubocop:disable Lint/ShadowedException
           warn("Ruby LSP Rails failed to initialize server: #{e.message}\n#{e.backtrace&.join("\n")}")
           warn("Server dependent features will not be available")

test/ruby_lsp_rails/runner_client_test.rb

@@ -39,10 +39,31 @@ class RunnerClientTest < ActiveSupport::TestCase
         assert_nil @client.model("ApplicationRecord") # ApplicationRecord is abstract
       end
 
+      test "falls back to null client when bin/rails is not found" do
+        FileUtils.mv("bin/rails", "bin/rails_backup")
+
+        assert_output("", %r{Ruby LSP Rails failed to locate bin/rails in the current directory}) do
+          client = RunnerClient.create_client
+
+          assert_instance_of(NullClient, client)
+          assert_nil(client.model("User"))
+          assert_predicate(client, :stopped?)
+        end
+      ensure
+        FileUtils.mv("bin/rails_backup", "bin/rails")
+      end
+
       test "failing to spawn server creates a null client" do
         FileUtils.mv("bin/rails", "bin/rails_backup")
+        File.open("bin/rails", "w") do |f|
+          f.write("foo")
+        end
+        File.chmod(0o755, "bin/rails")
 
-        assert_output("", %r{No such file or directory - bin/rails}) do
+        assert_output(
+          "",
+          %r{Ruby LSP Rails failed to initialize server: bin/rails: line 1: foo:( command)? not found},
+        ) do
           client = RunnerClient.create_client
 
           assert_instance_of(NullClient, client)