Replaces manual isinstance/<=0 checks in calculate_spread_ratio() and
determine_order_limits() with _valid_tick_price(), ensuring NaN, inf,
-inf, zero, negative, bool, and invalid-string tick values are
consistently rejected across all trading helpers.
Adds regression tests covering numeric-string acceptance and every
invalid-value category for both functions.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>