Thread executor's time_per_step into the trade Account freq (#1846)

[genisis0x]

Summary

`create_account_instance` always built `Account` with the hardcoded default `freq="day"`. When the user only has intraday data and runs the backtest with a higher-frequency executor (e.g. `time_per_step="60min"`), the benchmark fetch at `Account.init` tries to read day-level data and raises `ValueError: The benchmark [...] does not exist. Please provide the right benchmark` before the executor's `reset_common_infra` gets a chance to call `trade_account.reset(freq=time_per_step)`.

The fix derives the account's freq from the executor's `time_per_step` at the entry point and threads it through:

• `get_strategy_executor` reads `executor["kwargs"]["time_per_step"]` (or `executor.time_per_step` for an already-built executor) via a new tiny helper `_executor_time_per_step`.
• `create_account_instance` grows a `freq: str = "day"` parameter and passes it to `Account(freq=...)`.
• Falls back to the historic `"day"` default when the executor's freq cannot be determined (a Path or opaque object).

No change to the default day-frequency path: callers that don't set `time_per_step`, or that pass a Path-style executor, still get `freq="day"`.

Test plan

• New `tests/misc/test_backtest_account_freq_threading.py` covers the extraction helper:
  • dict executor with `time_per_step` → returns that value
  • dict executor without it → returns `None` (caller defaults to "day")
  • dict with non-string `time_per_step` is rejected (defensive)
  • executor instance with `time_per_step` attribute → returns it
  • executor instance without the attribute → returns `None`
  • Path/str executor → returns `None`

Fixes #1846

[genisis0x]

@microsoft-github-policy-service agree