fix: use timezone-aware UTC in web search example

[Ghraven]

Summary

Replace datetime.utcnow() in examples/web_search_gpt_oss_helper.py with datetime.now(timezone.utc).

Bug

The example helper builds Page.fetched_at timestamps with datetime.utcnow(), which is deprecated in Python 3.12 because it returns a naive datetime. Running the example on Python 3.12+ can emit deprecation warnings.

Before/After

Before:

fetched_at=datetime.utcnow()

After:

fetched_at=datetime.now(timezone.utc)

The value is still UTC, but now it is explicitly timezone-aware.

Verification

• python -m compileall -q examples/web_search_gpt_oss_helper.py
• git diff --check
• rg -n 'datetime\.utcnow\(' examples/web_search_gpt_oss_helper.py returns no matches