feat(V2): migrazione alpaca completa

Task 6.7: porting alpaca da services/mcp-alpaca a src/cerbero_mcp.
client.py + leverage_cap.py copiati 1:1 (default cap 1 cash).
tools.py: 17 tool senza ACL/Principal/audit. Router /mcp-alpaca con 18
route (env_info + 17 tool). Builder branch alpaca: paper=(env=="testnet"),
api_key viene da settings.alpaca.api_key_id. Test client + leverage_cap
migrati (15 test alpaca pass). Test builder con stub SDK alpaca-py.

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

tests/unit/exchanges/alpaca/conftest.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+import pytest
+from cerbero_mcp.exchanges.alpaca.client import AlpacaClient
+
+
+@pytest.fixture
+def mock_trading():
+    return MagicMock(name="alpaca_TradingClient")
+
+
+@pytest.fixture
+def mock_stock():
+    return MagicMock(name="alpaca_StockHistoricalDataClient")
+
+
+@pytest.fixture
+def mock_crypto():
+    return MagicMock(name="alpaca_CryptoHistoricalDataClient")
+
+
+@pytest.fixture
+def mock_option():
+    return MagicMock(name="alpaca_OptionHistoricalDataClient")
+
+
+@pytest.fixture
+def client(mock_trading, mock_stock, mock_crypto, mock_option):
+    return AlpacaClient(
+        api_key="test_key",
+        secret_key="test_secret",
+        paper=True,
+        trading=mock_trading,
+        stock_data=mock_stock,
+        crypto_data=mock_crypto,
+        option_data=mock_option,
+    )