Fix float rounding drift in order amount calculations

[Cassxbt]

Summary

• Use Decimal-based quantization for order-builder rounding helpers.
• Preserve cent-boundary sizes like 16.90, 33.30, and 66.60 when converting limit order amounts to token decimals.
• Add focused buy/sell regression coverage for 0.01 tick-size order amounts.

Motivation

Float arithmetic can round valid cent-boundary order sizes down before token decimal conversion. For example, a 16.90 size at 0.30 price can produce 5,067,000 instead of 5,070,000 maker amount, creating invalid sub-cent order amounts.

The Decimal rounding uses ROUND_HALF_UP for normal rounding to match the positive-value behavior of the TypeScript v2 client's Math.round path.

Closes #68.

Tests

• python -m pytest tests/order_builder/test_decimal_order_amounts.py tests/order_builder/test_helpers.py -q
• python -m pytest -q
• python -m black --check py_clob_client_v2/order_builder/helpers.py tests/order_builder/test_decimal_order_amounts.py
• git diff --check

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Note

Medium Risk
Touches core order amount rounding/conversion logic used to compute maker/taker amounts; subtle rounding behavior changes could affect order validity at certain tick sizes, but scope is contained and covered by new regression tests.

Overview
Switches order-builder rounding helpers from float/math-based rounding to Decimal.quantize with explicit ROUND_FLOOR/ROUND_CEILING/ROUND_HALF_UP, and updates to_token_decimals/decimal_places to avoid float drift when scaling to 6-decimal token units.

Adds regression tests ensuring limit buy/sell amount calculations at 0.01 tick size preserve cent-boundary sizes (e.g., 16.90 at 0.30) when converted to maker/taker token decimals.

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