Scrub invalid bytes when normalizing command output

When captured subprocess output contained a byte that isn't valid UTF-8,
normalize tagged the buffer as UTF-8 and then ran gsub over it, which
scans the whole string and raised Encoding::CompatibilityError: invalid
byte sequence in UTF-8. The error surfaced from inside the test harness
rather than on an assertion, so RSpec reported "Unable to find matching
line from backtrace" and the real output was lost.

Scrub the string after forcing the encoding so normalize never raises on
malformed bytes, and dup it first so forcing the encoding no longer
mutates the stored capture buffer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsbt

spec/support/command_execution.rb

@@ -72,7 +72,7 @@ def failure?
     attr_reader :failure_reason
 
     def normalize(string)
-      string.force_encoding(Encoding::UTF_8).strip.gsub("\r\n", "\n")
+      string.dup.force_encoding(Encoding::UTF_8).scrub.strip.gsub("\r\n", "\n")
     end
   end
 end