fix: handle missing minute-level data in get_higher_eq_freq_feature

[Whning0513]

This was the most involved fix of the three I worked on. #1854 causes a backtest crash when time_per_step='30min' is used — the error message says ValueError: can't find a freq from [Freq(30min)] that can resample to 1min!, which is confusing because the user never asked for 1min data.

I traced the crash from the backtest engine down through the call chain. PortfolioMetrics._cal_benchmark() calls D.features() with the 30min frequency. Since the CSI300 benchmark only has daily data, a ValueError is raised. The code then enters the minute-level fallback branch in get_higher_eq_freq_feature() (qlib/utils/resam.py), which hardcodes a retry with freq='1min' — but this fallback is not wrapped in try/except, unlike the day-level fallback earlier in the same function. When 1min data also doesn't exist, the raw exception propagates up as the misleading error above.

While reading the code, I also found a type inconsistency in Freq.get_recent_freq() (qlib/utils/time.py). The method builds a list of candidate frequencies — the first entry was stored as str(_freq) while subsequent entries kept the raw _freq (which could be either str or Freq). This made the return type unpredictable for callers like FileCalendarStorage._freq_file.

The fix spans two files:

1. qlib/utils/resam.py — Added a try/except (ValueError, KeyError) around the 1min fallback inside get_higher_eq_freq_feature(), matching the pattern already used by the day/week/month branch a few lines above. If 1min also fails, it falls through to day as the ultimate fallback.

2. qlib/utils/time.py — Wrapped all candidates in Freq.get_recent_freq() consistently with Freq(_freq), so the return type is always Freq | None regardless of which candidate wins.

Would appreciate a review, thanks!

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