fix: preserve binary multipart chunks

lib/openai/internal/util.rb

@@ -425,9 +425,9 @@ def close
         private def read_enum(max_len)
           case max_len
           in nil
-            @stream.to_a.join
+            @stream.to_a.map(&:b).join
           in Integer
-            @buf << @stream.next while @buf.length < max_len
+            @buf << @stream.next.b while @buf.length < max_len
             @buf.slice!(..max_len)
           end
         rescue StopIteration
@@ -476,7 +476,7 @@ def initialize(src, &blk)
             else
               src
             end
-          @buf = String.new
+          @buf = String.new.b
           @blk = blk
         end
       end

test/openai/internal/util_test.rb

@@ -330,6 +330,37 @@ def test_copy_read
     end
   end
 
+  def test_read_mixed_encoding_chunks
+    chunks = ["caf\u00E9", "\xFF\xFE".b]
+    expected = chunks.map(&:b).join
+
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(chunks.to_enum) do |chunk|
+      chunk
+    end
+    actual = String.new.b
+    loop do
+      chunk = adapter.read(2)
+      break unless chunk
+
+      actual << chunk
+    end
+
+    assert_equal(expected, actual)
+    assert_equal(Encoding::ASCII_8BIT, actual.encoding)
+  end
+
+  def test_read_all_mixed_encoding_chunks
+    chunks = ["caf\u00E9", "\xFF\xFE".b]
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(chunks.to_enum) do |chunk|
+      chunk
+    end
+
+    result = adapter.read
+
+    assert_equal(chunks.map(&:b).join, result)
+    assert_equal(Encoding::ASCII_8BIT, result.encoding)
+  end
+
   def test_copy_write
     cases = {
       StringIO.new => "",