fix: preserve explicit zero polling durations

[yigitcan-ozturk]

Checklist

• I have read both the CONTRIBUTING.md and Contributor License Agreement documents.
• I have created an issue or feature request and received approval from xAI maintainers. (minor changes like fixing typos can skip this step)
• I have tested my changes locally and they pass all CI checks.
• No documentation changes are required for this internal bug fix.

Description

PollTimer currently uses truthiness to apply defaults:

self._timeout = timeout or datetime.timedelta(minutes=10)
self._interval = interval or datetime.timedelta(seconds=1)

[yigitcan-ozturk]

Hi maintainers — just following up on this PR since it has been open for a little while. The change is intentionally small and remains scoped to preserving explicitly supplied zero polling durations, with regression coverage included. Please let me know if there is anything you'd like adjusted or any additional validation needed to move this forward. Thanks!