fix: normalize calendar list event start dates

[Success6666]

Summary

• normalize timezone-aware list_events start dates to UTC before sending timeMin
• keep naive ISO start dates treated as UTC to preserve current behavior
• add regression coverage for timezone-aware and naive start dates

Fixes #8670.

Tests

• .venv\Scripts\python.exe -m pytest libs\agno\tests\unit\tools\test_google_calendar.py -q
• .venv\Scripts\ruff.exe check libs\agno\agno\tools\google\calendar.py libs\agno\tests\unit\tools\test_google_calendar.py
• git diff --check

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.