fix: use timezone-aware cookbook timestamp

[Ghraven]

Summary

• Replace datetime.utcnow() in the dynamic tools cookbook with datetime.now(timezone.utc)
• Keep the example output explicitly UTC and timezone-aware

Closes #8060

Problem

The cookbook example returns the current time with datetime.utcnow().isoformat(). That produces a naive datetime and datetime.utcnow() is deprecated in Python 3.12.

Before / After

Before:

return datetime.utcnow().isoformat()

After:

return datetime.now(timezone.utc).isoformat()

The returned timestamp now includes UTC timezone information.

Verification

• python -m compileall -q cookbook\02_agents\15_dependencies\dynamic_tools.py
• git diff --check

This is a cookbook-only timestamp cleanup, so I did not run the full test suite.

[kausmeows]

Thanks for this @Ghraven!

[Ghraven]

Thank you for reviewing and merging this, I appreciate it!