Return the byte count from IO#write to a Windows console

`rb_w32_write_console` returns the number of UTF-16 code units it wrote, so writing "あ" to a console returned 1 instead of 3 as it does for files and NUL. Use the result only to tell success, as the other callers do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

io.c

@@ -2114,8 +2114,7 @@ io_fwrite(VALUE str, rb_io_t *fptr, int nosync)
 
 #ifdef _WIN32
     if (fptr->mode & FMODE_TTY) {
-        long len = rb_w32_write_console(str, fptr->fd);
-        if (len > 0) return len;
+        if (rb_w32_write_console(str, fptr->fd) > 0) return RSTRING_LEN(str);
     }
 #endif
 

test/ruby/test_io_m17n.rb

@@ -2656,6 +2656,19 @@ EOT
     end
   end if /mswin|mingw/ =~ RUBY_PLATFORM
 
+  def test_write_to_console
+    begin
+      con = File.open("CONOUT$", "r+")
+    rescue SystemCallError
+      omit "console is not available"
+    end
+    # non-ASCII, but leaves nothing visible on the console
+    str = "\u00a0\b"
+    assert_equal(str.bytesize, con.write(str))
+  ensure
+    con&.close
+  end if /mswin|mingw/ =~ RUBY_PLATFORM
+
   def test_cr_decorator_on_stdout
     with_pipe do |in_r, in_w|
       with_pipe do |out_r, out_w|