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.

tradingagents/default_config.py

@@ -97,7 +97,10 @@ DEFAULT_CONFIG = _apply_env_overrides({
         "oil commodities supply chain energy",
     ],
     # Data vendor configuration
-    # Category-level configuration (default for all tools in category)
+    # Category-level configuration (default for all tools in category).
+    # The configured value is the exact vendor chain — requests are NOT silently
+    # routed to vendors you didn't choose. For ordered fallback, list several,
+    # e.g. "yfinance,alpha_vantage". "default" uses all available vendors.
     "data_vendors": {
         "core_stock_apis": "yfinance",       # Options: alpha_vantage, yfinance
         "technical_indicators": "yfinance",  # Options: alpha_vantage, yfinance