Support newlines for the chatops "run" command

We ran into this as part of
https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1622. By
enabling the /m mode, the dot operator matches newlines. This allows the
use of newlines when using the "run" command.

changelogs/unreleased/chatops-multiline-messages.yml

+---
+title: Support newlines for the chatops "run" command
+merge_request: 56668
+author:
+type: changed

lib/gitlab/slash_commands/run.rb

@@ -5,7 +5,7 @@ module Gitlab
     # Slash command for triggering chatops jobs.
     class Run < BaseCommand
       def self.match(text)
-        /\Arun\s+(?<command>\S+)(\s+(?<arguments>.+))?\z/.match(text)
+        /\Arun\s+(?<command>\S+)(\s+(?<arguments>.+))?\z/m.match(text)
       end
 
       def self.help_message

spec/lib/gitlab/slash_commands/run_spec.rb

@@ -3,6 +3,26 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::SlashCommands::Run do
+  describe '.match' do
+    it 'returns true for a run command' do
+      expect(described_class.match('run foo')).to be_an_instance_of(MatchData)
+    end
+
+    it 'returns true for a run command with arguments' do
+      expect(described_class.match('run foo bar baz'))
+        .to be_an_instance_of(MatchData)
+    end
+
+    it 'returns true for a command containing newlines' do
+      expect(described_class.match("run foo\nbar\nbaz"))
+        .to be_an_instance_of(MatchData)
+    end
+
+    it 'returns false for an unrelated command' do
+      expect(described_class.match('foo bar')).to be_nil
+    end
+  end
+
   describe '.available?' do
     it 'returns true when builds are enabled for the project' do
       project = double(:project, builds_enabled?: true)