fix(example): use aware web search timestamps

[Ghraven]

Summary

• replace datetime.utcnow() in the GPT-OSS web search helper example with datetime.now(timezone.utc)
• keep the existing Page.fetched_at type and behavior while making timestamps timezone-aware

Problem

The example currently creates fetched-at timestamps with naive UTC datetimes. datetime.utcnow() is deprecated in modern Python, and the resulting values do not carry explicit timezone information.

Before / After

Before, fetched pages stored naive UTC timestamps.

After, fetched pages store UTC-aware timestamps with timezone.utc.

Verification

• python -m py_compile examples/web_search_gpt_oss_helper.py
• git diff --check

[vaderollama]

LGTM — replaces deprecated datetime.utcnow() with datetime.now(timezone.utc) in the web search example. Pure quality fix, no behavior change for users. Low risk.

@ParthSareen — ready to merge.