add ut coverage

[ceshihao]

…functionality

• Delete integration_test.go file containing extensive integration tests
• Remove tests for update session creation, searcher, downloader, installer, and agent info
• Eliminate history querying, searching, and server selection tests
• Remove property access and mutation tests for downloader and installer
• Delete tests for automatic updates, system information, and service manager
• Remove update collection and string collection tests
• Delete oleconv_test.go tests for IStringCollection nil dispatch and empty slice handling

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.43%. Comparing base (a5551a2) to head (ec2032b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #12       +/-   ##
===========================================
+ Coverage    5.60%   38.43%   +32.83%     
===========================================
  Files          34       34               
  Lines        1392     1392               
===========================================
+ Hits           78      535      +457     
+ Misses       1313      674      -639     
- Partials        1      183      +182     

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