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.

tradingagents/dataflows/utils.py

@@ -7,10 +7,12 @@ from typing import Annotated
 
 SavePathType = Annotated[str, "File path to save data. If None, data is not saved."]
 
-# Tickers can contain letters, digits, dot, dash, underscore, and caret
-# (for index symbols like ^GSPC). Anything else is rejected so the value
-# never escapes a containing directory when interpolated into a path.
-_TICKER_PATH_RE = re.compile(r"^[A-Za-z0-9._\-\^]+$")
+# Tickers can contain letters, digits, dot, dash, underscore, caret
+# (index symbols like ^GSPC), equals (futures like GC=F), and plus
+# (forex/CFD symbols like XAUUSD+). None of these enable directory
+# traversal, so the value never escapes a containing directory when
+# interpolated into a path. Anything else is rejected.
+_TICKER_PATH_RE = re.compile(r"^[A-Za-z0-9._\-\^=+]+$")
 
 
 def safe_ticker_component(value: str, *, max_len: int = 32) -> str: