fix(V2): Deribit _authenticate gestisce error envelope (no più KeyError 'result')

Quando Deribit risponde con {"error": {...}} su public/auth (creds errate,
scope mancante, env mismatch), il client esplodeva con KeyError: 'result' →
500 UNHANDLED_EXCEPTION sui tool privati (get_account_summary, get_positions).
Ora _authenticate solleva DeribitAuthError tipizzata, _request la converte
in error envelope coerente con il resto del flusso.

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

tests/unit/exchanges/deribit/test_client.py

@@ -154,6 +154,23 @@ async def test_get_account_summary(httpx_mock: HTTPXMock, client: DeribitClient)
     assert result["balance"] == 900.0
 
 
+@pytest.mark.asyncio
+async def test_private_call_with_bad_auth_returns_error_envelope(
+    httpx_mock: HTTPXMock, client: DeribitClient
+):
+    """Auth fallita (creds errate / scope mancante) → error envelope, non KeyError."""
+    httpx_mock.add_response(
+        url=re.compile(r"https://test\.deribit\.com/api/v2/public/auth"),
+        json={"error": {"code": 13004, "message": "invalid_credentials"}},
+        is_reusable=True,
+    )
+    summary = await client.get_account_summary("USDC")
+    assert summary["equity"] == 0
+    assert "invalid_credentials" in summary["error"]
+    positions = await client.get_positions("USDC")
+    assert positions == []
+
+
 @pytest.mark.asyncio
 async def test_place_order(httpx_mock: HTTPXMock, client: DeribitClient):
     httpx_mock.add_response(