Fix JRuby thread interrupt handling in pipelined get

Clear response buffer at start of pipelined_get to remove stale data
from interrupted operations. Uses clear() instead of reset() to maintain
proper pipeline state detection (pipeline_complete? remains true until
pipeline_response_setup is called).

This fixes intermittent test failures on JRuby where Thread#raise or
Thread#kill during get_multi could leave corrupted buffer data that
affected subsequent operations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: petergoldstein

lib/dalli/protocol/base.rb

@@ -222,6 +222,11 @@ def connect
       end
 
       def pipelined_get(keys)
+        # Clear buffer to remove any stale data from interrupted operations.
+        # Use clear (not reset) to keep pipeline_complete? = true, which is
+        # the expected state before pipeline_response_setup is called.
+        response_buffer.clear
+
         req = +''
         keys.each do |key|
           req << quiet_get_request(key)