Fix success comment detection in github-api.ts

[neubig]

This PR fixes an issue where success comments were not being properly detected in the github-api.ts file. The problem was that the string matching for success comments was too strict and did not properly handle the case where a PR was created.

Changes:

• Modified isSuccessComment to detect PR creation success comments by using a more flexible string match
• Added tests to verify comment detection behavior
• Fixed async cleanup issues in tests

Tested:

• Added unit tests for comment detection
• Manually verified that the cache is now properly built with success comments from issues #5363 and #5343

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