fix(V2): get_account_summary error path → numeric fields None invece di 0

Su errore di Deribit (auth fallita, ecc.) i campi equity/balance/margin/
available/unrealized_pnl/total_pnl ora sono None: signal chiaro di "valore
ignoto" vs "saldo realmente a zero". Risolve ambiguità lato client che
leggevano equity=0 senza accorgersi del campo error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/exchanges/deribit/client.py

@@ -321,12 +321,12 @@ class DeribitClient:
         r = raw.get("result")
         if not r:
             return {
-                "equity": 0,
-                "balance": 0,
-                "margin_balance": 0,
-                "available_funds": 0,
-                "unrealized_pnl": 0,
-                "total_pnl": 0,
+                "equity": None,
+                "balance": None,
+                "margin_balance": None,
+                "available_funds": None,
+                "unrealized_pnl": None,
+                "total_pnl": None,
                 "testnet": self.testnet,
                 "error": raw.get("error", "no result"),
             }

tests/unit/exchanges/deribit/test_client.py

@@ -165,7 +165,8 @@ async def test_private_call_with_bad_auth_returns_error_envelope(
         is_reusable=True,
     )
     summary = await client.get_account_summary("USDC")
-    assert summary["equity"] == 0
+    assert summary["equity"] is None
+    assert summary["balance"] is None
     assert "invalid_credentials" in summary["error"]
     positions = await client.get_positions("USDC")
     assert positions == []