Replace custom IO object with a transport manager IO

app/models/zip_file.rb

@@ -39,9 +39,15 @@ class ZipFile
         )
 
         writer = Writer.new
-        service.upload(blob.key, writer.io) do |io|
-          writer.stream_to(io)
+
+        # Use S3's upload_stream directly for write-based streaming.
+        # ActiveStorage's upload method expects a read-based IO, but ZipKit
+        # needs a write-based stream. The TransferManager's upload_stream
+        # yields a writable IO that we can stream directly to.
+        service.send(:upload_stream, key: blob.key, part_size: 100.megabytes) do |write_stream|
+          writer.stream_to(write_stream)
           yield writer
+          writer.close
         end
 
         blob.update!(byte_size: writer.byte_size, checksum: writer.checksum)

app/models/zip_file/writer.rb

@@ -9,10 +9,6 @@ class ZipFile::Writer
     @digest = Digest::MD5.new
   end
 
-  def io
-    @io ||= ZipFile::Writer::IO.new(self)
-  end
-
   def stream_to(io)
     @output_io = io
   end

app/models/zip_file/writer/io.rb

@@ -1,18 +0,0 @@
-class ZipFile::Writer::IO
-  def initialize(writer)
-    @writer = writer
-  end
-
-  def write(data)
-    @writer.write(data)
-  end
-
-  def <<(data)
-    write(data)
-    self
-  end
-
-  def size
-    @writer.byte_size
-  end
-end

test/models/zip_file_test.rb

@@ -1,13 +1,6 @@
 require "test_helper"
 
 class ZipFileTest < ActiveSupport::TestCase
-  test "writer io exposes size for S3 multipart upload detection" do
-    writer = ZipFile::Writer.new
-
-    assert_respond_to writer.io, :size
-    assert_equal writer.byte_size, writer.io.size
-  end
-
   test "writer adds files with content" do
     tempfile = Tempfile.new([ "test", ".zip" ])
     tempfile.binmode