Extract detached-blob guard into its own file

Separate the attachment existence check from the broadcast suppression
override so each file has a single responsibility. The guard now lives
in ActiveStorageAnalyzeJobSkipDetached with its own documentation
explaining the upload-then-delete race condition.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/lib/rails_ext/active_storage_analyze_job_skip_detached_test.rb

@@ -1,6 +1,6 @@
 require "test_helper"
 
-class ActiveStorageAnalyzeJobSuppressBroadcastsTest < ActiveSupport::TestCase
+class ActiveStorageAnalyzeJobSkipDetachedTest < ActiveSupport::TestCase
   test "skips analysis when blob has no attachments" do
     blob = ActiveStorage::Blob.create_and_upload!(
       io: StringIO.new("x" * 1024), filename: "orphan.txt", content_type: "text/plain"