fix(py): validate account usage timeouts before Duration conversion

[upsetbit]

Summary

• convert Python account usage timeouts with the non-panicking Duration API
• surface invalid values as typed invalidRequest errors before starting Codex
• cover synchronous and asyncio callers for negative, NaN, infinite, and oversized finite values

Testing

• UV_VENV_CLEAR=1 just ci
• 1,000 synchronous and 1,000 asyncio adversarial timeout calls

Closes #19