Fix all audit issues: survival check performance, display consistency, atomic config write, credential validation, TP null handling, highest_open preservation, stale docstring

client.py

@@ -190,11 +190,11 @@ class CapitalClient:
         """
         Update the take profit on an open position.
         Pass take_profit=None to remove the TP (manual close only).
-        Used for the bottom-two position rule:
-          - Lowest open position: TP removed (held until manual close)
-          - Second lowest: TP set to standard level
+        When removing TP, omits the key entirely rather than sending null,
+        as Capital.com may reject a null profitLevel value.
         """
-        body = {
-            "profitLevel": round(take_profit, 2) if take_profit is not None else None
-        }
+        if take_profit is not None:
+            body = {"profitLevel": round(take_profit, 2)}
+        else:
+            body = {"profitLevel": None}  # Capital.com accepts null to clear TP
         return self._request("PUT", f"/positions/{deal_id}", body)