fix: support commodity/forex/crypto tickers and never invent prices (#781)

Analyzing a symbol Yahoo Finance does not recognize (e.g. XAUUSD+) could
produce an invented price instead of an error. The agent now either prices
the correct instrument or clearly reports that data is unavailable.

Ticker support:
- Commodities/forex/crypto resolve to the symbol Yahoo actually serves, so
  you can enter the common form and it just works:
    XAUUSD / XAUUSD+ / GOLD  -> GC=F   (gold)
    USOIL                    -> CL=F   (WTI crude)
    EURUSD                   -> EURUSD=X
    BTCUSD                   -> BTC-USD
    SPX500 / NAS100          -> ^GSPC / ^NDX
  Native Yahoo symbols (AAPL, GC=F, ^GSPC) keep working unchanged. New
  instruments are added by extending the alias table.

Reliability:
- Unknown or delisted symbols now return a clear "data unavailable" result
  the agent reports verbatim, instead of a value the model fills in.
- A failed fetch no longer leaves a broken symbol cached until the cache is
  cleared by hand.

tests/test_no_data_handling.py

@@ -0,0 +1,88 @@
+"""Tests that empty vendor results never become fabricated data.
+
+Covers two systematic fixes:
+  - load_ohlcv must not cache an empty download (cache poisoning), and must
+    raise NoMarketDataError instead of returning an empty frame.
+  - route_to_vendor must convert NoMarketDataError into a single explicit
+    "NO_DATA_AVAILABLE" sentinel after all vendors are exhausted.
+"""
+
+import os
+import unittest
+from unittest import mock
+
+import pandas as pd
+import pytest
+
+from tradingagents.dataflows import stockstats_utils, interface
+from tradingagents.dataflows.config import set_config
+from tradingagents.dataflows.symbol_utils import NoMarketDataError
+
+
+@pytest.mark.unit
+class TestLoadOhlcvNoPoison(unittest.TestCase):
+    def setUp(self):
+        self._tmp = os.path.join(os.path.dirname(__file__), "_tmp_cache")
+        os.makedirs(self._tmp, exist_ok=True)
+        set_config({"data_cache_dir": self._tmp})
+
+    def tearDown(self):
+        for f in os.listdir(self._tmp):
+            os.remove(os.path.join(self._tmp, f))
+        os.rmdir(self._tmp)
+
+    def test_empty_download_raises_and_does_not_cache(self):
+        empty = pd.DataFrame()
+        with mock.patch.object(stockstats_utils.yf, "download", return_value=empty) as dl:
+            with self.assertRaises(NoMarketDataError):
+                stockstats_utils.load_ohlcv("FAKE", "2026-01-01")
+        # Nothing should have been written to the cache.
+        self.assertEqual(os.listdir(self._tmp), [])
+
+        # A second call must re-attempt the fetch (no poisoned cache served).
+        with mock.patch.object(stockstats_utils.yf, "download", return_value=empty) as dl2:
+            with self.assertRaises(NoMarketDataError):
+                stockstats_utils.load_ohlcv("FAKE", "2026-01-01")
+            self.assertTrue(dl2.called)
+
+
+@pytest.mark.unit
+class TestRouteToVendorSentinel(unittest.TestCase):
+    def test_no_data_from_all_vendors_returns_sentinel(self):
+        def raises_no_data(symbol, *a, **k):
+            raise NoMarketDataError(symbol, "GC=F", "no rows")
+
+        patched = {"yfinance": raises_no_data, "alpha_vantage": raises_no_data}
+        with mock.patch.dict(
+            interface.VENDOR_METHODS, {"get_stock_data": patched}, clear=False
+        ):
+            result = interface.route_to_vendor(
+                "get_stock_data", "XAUUSD+", "2026-01-01", "2026-01-10"
+            )
+        self.assertIn("NO_DATA_AVAILABLE", result)
+        self.assertIn("XAUUSD+", result)
+        self.assertIn("GC=F", result)
+        self.assertIn("Do not estimate", result)
+
+    def test_unconfigured_fallback_does_not_mask_no_data(self):
+        # When the primary vendor reports no data and the fallback is simply
+        # unavailable (e.g. missing API key -> raises), the no-data sentinel
+        # must win rather than the fallback's incidental error crashing out.
+        def raises_no_data(symbol, *a, **k):
+            raise NoMarketDataError(symbol, symbol, "no rows")
+
+        def raises_unavailable(symbol, *a, **k):
+            raise ValueError("ALPHA_VANTAGE_API_KEY environment variable is not set.")
+
+        patched = {"yfinance": raises_no_data, "alpha_vantage": raises_unavailable}
+        with mock.patch.dict(
+            interface.VENDOR_METHODS, {"get_stock_data": patched}, clear=False
+        ):
+            result = interface.route_to_vendor(
+                "get_stock_data", "FAKE", "2026-01-01", "2026-01-10"
+            )
+        self.assertIn("NO_DATA_AVAILABLE", result)
+
+
+if __name__ == "__main__":
+    unittest.main()