fix: preserve exploration tool history and cap context budget

tests/unit/admin_console/test_session_usage_endpoint.py

@@ -80,7 +80,7 @@ def test_usage_sums_all_agents_and_tracks_latest_operator_context(tmp_path):
     assert usage["cached_tokens"] == 90_000
     assert usage["operator_context_tokens"] == 150_000
     assert usage["operator_context_updated_at"] == 13.0
-    assert usage["operator_context_window_tokens"] == 1_000_000
+    assert usage["operator_context_window_tokens"] == 300_000
     assert usage["profile"] == "pro"
     assert usage["run_tuning"] == {"verification_level": "checkpoints", "explorer_mode": "pro"}