fix(trend): use mark price for DCA trigger on gate_bot

Poll trend plans with mark price (same as UI) instead of ticker last, add get_symbol_mark_price to gate_bot, tolerate position API blips, and log DCA order failures.

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

tests/test_trend_dca_pnl.py

@@ -17,6 +17,12 @@ def test_trend_dca_short_not_before_first_level():
     assert trend_dca_level_reached(direction, 0.3413, 0.3413)
 
 
+def test_trend_dca_long_mark_below_trigger():
+    direction = "long"
+    assert trend_dca_level_reached(direction, 0.344, 0.3465)
+    assert not trend_dca_level_reached(direction, 0.347, 0.3465)
+
+
 def test_trend_effective_margin_first_leg_only():
     plan = {
         "plan_margin_capital": 12.11,