fix: apply KR session-aware exchange routing for rankings and orders (#409)

tests/test_kr_exchange_router.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from src.broker.kr_exchange_router import KRExchangeRouter
+
+
+def test_ranking_market_code_by_session() -> None:
+    router = KRExchangeRouter()
+    assert router.resolve_for_ranking("KRX_REG") == "J"
+    assert router.resolve_for_ranking("NXT_PRE") == "NX"
+    assert router.resolve_for_ranking("NXT_AFTER") == "NX"
+
+
+def test_order_exchange_falls_back_to_session_default_on_missing_data() -> None:
+    router = KRExchangeRouter()
+    resolved = router.resolve_for_order(
+        stock_code="0001A0",
+        session_id="NXT_PRE",
+        is_dual_listed=True,
+        spread_krx=None,
+        spread_nxt=None,
+        liquidity_krx=None,
+        liquidity_nxt=None,
+    )
+    assert resolved.exchange_code == "NXT"
+    assert resolved.reason == "fallback_data_unavailable"
+
+
+def test_order_exchange_uses_spread_preference_for_dual_listing() -> None:
+    router = KRExchangeRouter()
+    resolved = router.resolve_for_order(
+        stock_code="0001A0",
+        session_id="KRX_REG",
+        is_dual_listed=True,
+        spread_krx=0.005,
+        spread_nxt=0.003,
+        liquidity_krx=100000.0,
+        liquidity_nxt=90000.0,
+    )
+    assert resolved.exchange_code == "NXT"
+    assert resolved.reason == "dual_listing_spread"