Add actionable advice for backtest range limit errors - #142

Summary

• add recommended start/end dates to validate_backtest_range errors
• include the recommended date window in the 400 response message so clients do not have to show a generic Request failed with status code 400
• add regression coverage for the Forex 15m long-range case from Backtest's Date Range too long can cause Request failed with status code 400 #130 , including indicator warmup bars

Related to #130.

Tests

• .\.venv\Scripts\python -m pytest tests\test_backtest_limits.py -q
• .\.venv\Scripts\python -m pytest tests\test_backtest_presets.py tests\test_backtest_limits.py -q
• git diff --check