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_safe_ticker_component.py

@@ -14,6 +14,11 @@ class TestSafeTickerComponent(unittest.TestCase):
         for ticker in ("AAPL", "BRK-B", "BRK.A", "0700.HK", "7203.T", "BHP.AX", "^GSPC"):
             self.assertEqual(safe_ticker_component(ticker), ticker)
 
+    def test_accepts_futures_and_forex_formats(self):
+        # Futures use '=' (GC=F gold, CL=F crude), forex/CFD symbols use '+'.
+        for ticker in ("GC=F", "CL=F", "ES=F", "XAUUSD+", "EURUSD+"):
+            self.assertEqual(safe_ticker_component(ticker), ticker)
+
     def test_rejects_path_separators(self):
         for bad in (".", "..", "../etc", "a/b", "a\\b", "/abs", "..\\..\\x"):
             with self.assertRaises(ValueError):