fix(data): respect the configured vendor chain and log vendor failures

The router silently extended every request to all available vendors regardless
of config, so an explicit single-vendor choice still fell back to others and
returned data from an unexpected source (#988, #289), and serious primary-vendor
errors were swallowed without a trace (#989). The configured vendor list is now
the exact chain (list several for ordered fallback; "default" uses all), unknown
vendors raise, and swallowed vendor errors are logged. Adds an autouse config
isolation fixture so vendor config can't leak between tests.

tests/conftest.py

@@ -35,6 +35,25 @@ def _dummy_api_keys(monkeypatch):
         monkeypatch.setenv(env_var, os.environ.get(env_var, "placeholder"))
 
 
+@pytest.fixture(autouse=True)
+def _isolate_config():
+    """Reset the global dataflows config before and after each test.
+
+    ``set_config`` merges (it never clears keys absent from the override), so a
+    test that sets e.g. ``tool_vendors`` would otherwise leak into later tests
+    and make routing behavior order-dependent. Replace the global outright so
+    every test starts from a clean DEFAULT_CONFIG.
+    """
+    import copy
+
+    import tradingagents.dataflows.config as config_module
+    import tradingagents.default_config as default_config
+
+    config_module._config = copy.deepcopy(default_config.DEFAULT_CONFIG)
+    yield
+    config_module._config = copy.deepcopy(default_config.DEFAULT_CONFIG)
+
+
 @pytest.fixture()
 def mock_llm_client():
     client = MagicMock()