fix: invalidate stale 12-item volume rank cache and force full top20 refresh

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_hub_volume_rank_lib.py

@@ -2,6 +2,8 @@ from datetime import datetime
 
 from hub_volume_rank_lib import (
     CACHE_VERSION,
+    TOP_N_DEFAULT,
+    _exchange_rank_row_stale,
     _okx_turnover_usdt,
     cache_needs_refresh,
     format_volume_quote,
@@ -53,3 +55,11 @@ def test_cache_needs_refresh_and_merge():
 def test_stale_cache_version_forces_refresh():
     cache = {"version": CACHE_VERSION - 1, "rank_date": "2026-06-07", "exchanges": {"okx": {"items": [{}]}}}
     assert cache_needs_refresh(cache) is True
+
+
+def test_short_item_list_is_stale():
+    items = [{"rank": i, "symbol": f"S{i}/USDT"} for i in range(1, 13)]
+    row = {"items": items, "total_symbols": 12}
+    assert _exchange_rank_row_stale(row) is True
+    full = {"items": items + [{"rank": i, "symbol": f"X{i}/USDT"} for i in range(13, TOP_N_DEFAULT + 1)], "total_symbols": 300}
+    assert _exchange_rank_row_stale(full) is False