Hand FakeReadPartialIO binary chunks

replace copies the encoding along with the content, so UTF-8 literals
turned @rbuf into UTF-8 for the whole test. A real IO leaves the binary
buffer it was given alone, and readuntil mixes byte counts with
String#index offsets that only agree while the buffer is binary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

test/net/protocol/test_protocol.rb

@@ -130,7 +130,9 @@ def test_write0_timeout_multi2
 
   class FakeReadPartialIO
     def initialize(chunks)
-      @chunks = chunks.map(&:dup)
+      # Binary, like the bytes a real IO hands back, and duped because
+      # rbuf_fill clears whatever read_nonblock returns.
+      @chunks = chunks.map { |chunk| chunk.b }
     end
 
     def read_nonblock(size, buf = nil, exception: false)