1. Fix Friday logic edge case comment
- Clarified get_default_backtest_dates() docstring
- Removed confusing 'at least a week old' comment
- Explicitly documented Friday behavior
2. Reorder validation checks in validate_date_range_for_symbol()
- Now checks start > end first (most fundamental)
- Then checks end > today (future dates)
- Finally checks symbol-specific bounds
- Users get most actionable error first
3. Add server-side numeric bounds validation
- New validate_numeric_inputs() function
- Validates units > 0, loan >= 0, 0 < LTV < 1
- Called in run_backtest() before service call
4. Add boundary tests
- Test start_date exactly at SYMBOL_MIN_DATES boundary
- Test same-day date range (start == end)
- Test end_date exactly today
- Test end_date tomorrow (future)
- Test validation order returns most actionable error
- Test near-zero and large values for units calculation
- Test LTV at boundaries (0, 1, 0.01, 0.99)
5. Add tests for validate_numeric_inputs
- Valid inputs, zero/negative values
- LTV boundary conditions
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