PR #6240 changed tail protection from protect_last_n to min(3, ...)
which increased the minimum compressible message count and shifted
tail boundaries. Three tests broke:
- test_summary_role_avoids_consecutive_user_messages: 6→8 msgs
- test_double_collision_user_head_assistant_tail: 7→8 msgs
- test_no_collision_scenarios_still_work: 6→8 msgs
All tests now exceed the new min_for_compress threshold (6) and
maintain proper role alternation in both head and tail sections.
BongSuCHOI
c506126123