test(session-log): pin no-session_json regression + drop trailing whitespace

Adds TestNoSessionJsonSnapshot to lock the contract that session_log_file
attribute, _save_session_log method, and the per-session JSON snapshot
writer are gone. logs_dir is retained for request_dump_*.json.

Also cleans up stray trailing whitespace in test_run_agent_codex_responses
introduced when the _save_session_log stub line was deleted.

tests/run_agent/test_run_agent_codex_responses.py

@@ -362,7 +362,7 @@ def test_build_api_kwargs_codex_preserves_supported_efforts(monkeypatch):
         agent._cleanup_task_resources = lambda task_id: None
         agent._persist_session = lambda messages, history=None: None
         agent._save_trajectory = lambda messages, user_message, completed: None
-    
+
         kwargs = agent._build_api_kwargs(
             [
                 {"role": "system", "content": "sys"},