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)