fix(example): use timezone-aware web search timestamps

[Ghraven]

Problem

examples/web_search_gpt_oss_helper.py stores page fetch timestamps with datetime.utcnow(). That creates naive datetime objects even though these values represent UTC.

Before / after

Before this change, helper pages had naive UTC-looking fetched_at values.

After this change, the helper uses datetime.now(timezone.utc) for search, fetched page, and find-result page timestamps, so fetched_at is explicitly UTC-aware.

Verification

• python -m py_compile examples/web_search_gpt_oss_helper.py
• direct smoke check constructing a search results page and asserting fetched_at has a UTC offset
• git diff --check