lint: drop redundant unused import in regeneration test (#6826)

Removes the local re-import of api.streaming._session_payload_with_full_messages
inside test_in_tail_duplicate_guard_refuses_bounded_and_full_revision_accepted;
it is already imported at module level (line 17) and unused in the function body,
tripping F401 on CI's hosted lint. Mechanical maintainer fix per auto-fix policy.

Co-authored-by: webtecnica <webtecnica@users.noreply.github.com>

tests/test_issue6611_regeneration_authority.py

@@ -890,7 +890,6 @@ def test_in_tail_duplicate_guard_refuses_bounded_and_full_revision_accepted(monk
         regeneration_revision_for,
         regeneration_state,
     )
-    from api.streaming import _session_payload_with_full_messages
 
     # 212 rows (above the 200-row anchor budget); the user row at index 210
     # repeats the visible key of the user row at index 100 — a duplicate that