Bugfix: 修复SqlSessionService在sqlite下因时区设置不对导致频繁warn

[weimch]

• 问题：当前在SqlSessionService的实现中，默认创建DB的表中，update_time使用了sqlalchemy的func.now，但在更新时间时，使用了datetime.now，在sqlite实现里，func.now默认使用了utc的时间，而datetime.now不是utc的时间，导致append_event时，因时区不同，导致diff失败，warn告警
• 解决方案：总是使用func.now来更新时间

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@e296c88). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
trpc_agent_sdk/sessions/_sql_session_service.py	90.00000%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main         #73   +/-   ##
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  Coverage        ?   87.32279%           
==========================================
  Files           ?         413           
  Lines           ?       40135           
  Branches        ?           0           
==========================================
  Hits            ?       35047           
  Misses          ?        5088           
  Partials        ?           0           

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