yfinance intermittently returns a year-old partial frame (e.g. June 2025 rows
for a June 2026 request) that still has rows and a Close, so it passed the
empty-check and silently fed a wrong close price and indicators into the report
(#1021). Add a freshness guard that rejects a frame whose latest row is far
older than the requested date, on both the raw OHLCV path and the indicator
path. It raises the existing NoMarketDataError with a stale-specific detail, so
the vendor router's try-next-vendor and single unavailable-signal handling apply
unchanged; the sentinel now surfaces that detail so the agent reports the
specific reason rather than fabricating a value.
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