test: pin the sink against silent read failures

Greptile flagged the JS counterpart for reading every failure as an empty
sink, which lets the deduplication phase forget the effect id and still
pass. The Ruby sink already rescues only Errno::ENOENT, and the Ruby
effect worker already fails when no effect became claimable, so neither
defect exists here.

Both properties were incidental until now. These two tests make them
deliberate: a truncated JSON file and a path that is a directory must
raise rather than read as a first run. Widening the rescue to
StandardError fails both.

Author: cardmagic

test/unit/at_least_once_sink_test.rb

@@ -42,4 +42,19 @@ class AtLeastOnceSinkTest < ActiveSupport::TestCase
       assert_equal [], AtLeastOnceSink.read(File.join(directory, "missing.json"))
     end
   end
+
+  test "refuses to read a damaged sink as an empty one" do
+    Dir.mktmpdir do |directory|
+      path = File.join(directory, "sink.json")
+      File.write(path, "{ deliveries: ")
+
+      assert_raises(JSON::ParserError) { AtLeastOnceSink.read(path) }
+    end
+  end
+
+  test "refuses to read an unreadable sink as an empty one" do
+    Dir.mktmpdir do |directory|
+      assert_raises(Errno::EISDIR) { AtLeastOnceSink.read(directory) }
+    end
+  end
 end