Make sure TraceReader uses Encoding.default_external

Encoding.default_external was chosen over
Encoding.default_internal because File.read is
returning Encoding.default_external, therefore
we should align with it. Alternatively, we could
force both of them to be Encoding.default_internal.

However, ideally this should be determined by different
projects. For example, some projects might want to use
an encoding different to what GitLab is using.

This might not happen soon though.

Closes #27052

lib/gitlab/ci/trace_reader.rb

@@ -42,6 +42,7 @@ module Gitlab
           end
 
           chunks.join.lines.last(max_lines).join
+            .force_encoding(Encoding.default_external)
         end
       end
     end

spec/lib/gitlab/ci/trace_reader_spec.rb

@@ -11,13 +11,25 @@ describe Gitlab::Ci::TraceReader do
       last_lines = random_lines
 
       expected = lines.last(last_lines).join
+      result = subject.read(last_lines: last_lines)
 
-      expect(subject.read(last_lines: last_lines)).to eq(expected)
+      expect(result).to eq(expected)
+      expect(result.encoding).to eq(Encoding.default_external)
     end
   end
 
   it 'returns everything if trying to get too many lines' do
-    expect(build_subject.read(last_lines: lines.size * 2)).to eq(lines.join)
+    result = build_subject.read(last_lines: lines.size * 2)
+
+    expect(result).to eq(lines.join)
+    expect(result.encoding).to eq(Encoding.default_external)
+  end
+
+  it 'returns all contents if last_lines is not specified' do
+    result = build_subject.read
+
+    expect(result).to eq(lines.join)
+    expect(result.encoding).to eq(Encoding.default_external)
   end
 
   it 'raises an error if not passing an integer for last_lines' do