refactor(mcp-common): centralize base_url defaults in resolve_environment

Aggiunti kwargs opzionali default_base_url_live / default_base_url_testnet
a resolve_environment. Rimosse 8 chiamate creds.setdefault duplicate dai
4 servizi (alpaca, bybit, deribit, hyperliquid) ora passano gli URL
canonici direttamente al resolver.

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

services/common/tests/test_environment.py

@@ -74,6 +74,37 @@ def test_env_value_truthy_parsing(monkeypatch, raw, expected):
     assert info.environment == expected
 
 
+def test_default_base_urls_applied_when_creds_missing(monkeypatch):
+    monkeypatch.delenv("X_TESTNET", raising=False)
+    creds: dict = {}
+    info = resolve_environment(
+        creds,
+        env_var="X_TESTNET",
+        flag_key="testnet",
+        exchange="x",
+        default_base_url_live="https://live.example",
+        default_base_url_testnet="https://test.example",
+    )
+    assert info.base_url == "https://test.example"
+    assert creds["base_url_live"] == "https://live.example"
+    assert creds["base_url_testnet"] == "https://test.example"
+
+
+def test_creds_base_urls_override_defaults(monkeypatch):
+    monkeypatch.delenv("X_TESTNET", raising=False)
+    creds = {"base_url_live": "L", "base_url_testnet": "T"}
+    info = resolve_environment(
+        creds,
+        env_var="X_TESTNET",
+        flag_key="testnet",
+        exchange="x",
+        default_base_url_live="https://live.example",
+        default_base_url_testnet="https://test.example",
+    )
+    assert info.base_url == "T"
+    assert creds["base_url_live"] == "L"
+
+
 def test_alpaca_paper_flag_key(monkeypatch):
     """Alpaca usa 'paper' invece di 'testnet' nel secret."""
     monkeypatch.delenv("ALPACA_PAPER", raising=False)