Delete old /test help messages after a successful invocation

[danwinship]

If a user misuses /test or /retest and gets a 50-page-long help message from prow, and then triggers a test correctly afterward, delete the help message so as to de-spam the PR.

Fixes #374

I don't know this codebase at all and probably did stuff wrong. Also, I'm not sure how to actually test it. (I assume I need to add a unit test to pkg/plugins/trigger/ too, but I was having a hard time figuring out if I could actually figure out which test cases should result in comment deletion correctly just from the info already in the testcase struct, so I gave up on it for now since you might request large rewrites anyway...)

/assign @petr-muller
since you 👍 'd the idea on #374...

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: danwinship
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[petr-muller]

I'll let GH copilot review this as a resource but so far I do not have great experience with the result so feel free to just ignore whatever it says unless its a clear slam dunk.

[Copilot]

Pull Request Overview

This PR implements functionality to automatically delete old help messages from the /test and /retest commands after a successful test invocation, preventing spam in pull requests when users initially misuse the commands and then trigger tests correctly.

Key changes:

• Added ShouldRespondByPruningHelp and IsHelpComment functions to identify when to prune help comments
• Modified handleGenericComment to accept a comment pruner and call it when appropriate
• Updated test infrastructure to validate comment pruning behavior

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/plugins/trigger/trigger.go	Modified to get comment pruner and pass it to handleGenericComment
pkg/plugins/trigger/generic-comment.go	Added commentPruner interface and logic to prune help comments on successful triggers
pkg/plugins/trigger/generic-comment_test.go	Added fake comment pruner and test validation for pruning behavior
pkg/pjutil/help.go	Added functions to identify help comments and determine when to prune them
pkg/pjutil/help_test.go	Added comprehensive tests for help comment identification and pruning logic

Comments suppressed due to low confidence (1)

pkg/pjutil/help.go:1

• Incorrect variable used in optional test commands section. Should use optionalTestCommands instead of requiredTestCommands.

/*

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[danwinship]

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Ship it! 😎

(I'm not sure what the "Comments suppressed due to low confidence" is even referring to? It seems like it hallucinated a non-existent incorrect comment and then explained how to fix it and then realized that it couldn't find the comment it was trying to explain how to fix?)