{server/agui, examples}: support activity message for history

[Flash-LHR]

前置PR：ag-ui-protocol/ag-ui#725

[Flash-LHR]

前置PR：ag-ui-protocol/ag-ui#725

前置 PR 已合入

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.87854%. Comparing base (4fd216f) to head (13e1adb).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #766         +/-   ##
===================================================
+ Coverage   87.84758%   87.87854%   +0.03096%     
===================================================
  Files            287         287                 
  Lines          37186       37248         +62     
===================================================
+ Hits           32667       32733         +66     
+ Misses          2900        2898          -2     
+ Partials        1619        1617          -2     

Flag	Coverage Δ
unittests	87.87854% <100.00000%> (+0.03096%)	⬆️

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