fix: preserve explicit zero order salt

[korowant]

What changed

• preserve an explicitly supplied salt=0 when building an order
• add regression coverage for the zero-salt case

Why

BuildOrderInput.salt accepts integers, including zero, but data.salt or self._generate_salt() treated zero as if no salt had been supplied. The builder then substituted a generated salt, changing the serialized order and its typed-data hash from the caller's intent.

The new explicit None check distinguishes an omitted salt from a valid zero value.

Impact

Callers can reliably build deterministic orders with a zero salt. Existing behavior for omitted and non-zero salts is unchanged.

Validation

• PYTHONPATH=src pytest -q — 134 passed
• ruff check src/predict_sdk/order_builder.py tests/test_order_builder.py — passed
• git diff --check — passed