refactor(V2): alpaca client da alpaca-py a httpx puro (parità V1)

Riscrive `AlpacaClient` su `httpx.AsyncClient` rimuovendo ogni dipendenza
runtime da `alpaca-py`. 4 endpoint base distinti (trading paper/live,
stock data, crypto data, options data) gestiti via helper `_request`
con header `APCA-API-KEY-ID` / `APCA-API-SECRET-KEY`. Firma costruttore
e attributi pubblici (`paper`, `base_url`) invariati; `base_url` override
applica al solo trading endpoint. Nuovo `aclose()` per cleanup connessioni.

Test riscritti su `pytest-httpx` (29 test alpaca + leverage cap).

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

tests/unit/exchanges/alpaca/conftest.py

@@ -1,39 +1,23 @@
 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")
+async def client():
+    """AlpacaClient paper su httpx mock (gestito da pytest-httpx)."""
+    c = AlpacaClient(api_key="test_key", secret_key="test_secret", paper=True)
+    try:
+        yield c
+    finally:
+        await c.aclose()
 
 
 @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,
-    )
+async def client_live():
+    c = AlpacaClient(api_key="test_key", secret_key="test_secret", paper=False)
+    try:
+        yield c
+    finally:
+        await c.aclose()