Fix ClassCastException when SCMSource is not GitHubSCMSource

[kovalevsky]

Problem

The getKey() method in IssueCommentTrigger, LabelAddedTrigger, and PullRequestReviewTrigger performs an unsafe cast of the result of SCMSource.SourceByItem.findSource() to GitHubSCMSource:

final GitHubSCMSource scmSource = (GitHubSCMSource) SCMSource.SourceByItem.findSource(project);

This throws a ClassCastException when the source is a NullSCMSource (or any other non-GitHub SCM source):

java.lang.ClassCastException: class jenkins.scm.impl.NullSCMSource cannot be cast to class org.jenkinsci.plugins.github_branch_source.GitHubSCMSource
    at IssueCommentTrigger.getKey(IssueCommentTrigger.java:65)
    at IssueCommentTrigger.stop(IssueCommentTrigger.java:58)
    at PipelineTriggersJobProperty.stopTriggers(PipelineTriggersJobProperty.java:106)
    at WorkflowJob.addProperty(WorkflowJob.java:199)
    at BranchProjectFactory.decorate(BranchProjectFactory.java:255)
    at MultiBranchProject$SCMEventListenerImpl.processHeadUpdate(MultiBranchProject.java:1685)

Impact

This happens during the stop()/start() cycle triggered by MultiBranchProject.processHeadUpdate() when a push event causes BranchProjectFactory.decorate() to call stopTriggers(). The exception in stop() prevents the trigger from being re-registered in start(), silently removing the job from the DescriptorImpl.jobs map. Subsequent webhook events (issue comments, labels, PR reviews) for that PR are then silently ignored because the key lookup returns an empty set.

This is particularly hard to diagnose because:

• The webhook returns HTTP 200
• No error is logged at the time the webhook is processed (the handleIssueComment method just silently returns when the jobs map has no entry for the key)
• The ClassCastException happens earlier during the push event processing, not during the comment event

Fix

Added instanceof checks before casting in getKey(), returning null when the source is not a GitHubSCMSource. Callers in start() and stop() now guard against null keys. A warning is logged when a non-GitHub SCM source is encountered to help diagnose similar issues.

The same fix is applied to all three affected trigger classes:

• IssueCommentTrigger
• LabelAddedTrigger
• PullRequestReviewTrigger