Throw an exception when trying to save actions for a FlowNode for a completed execution

[dwnusbaum]

See jenkinsci/pipeline-build-step-plugin#128 (comment) and the next comment.

If FlowExecution.saveActions is called for a Pipeline with a completed execution, in some cases (all cases as of #807), the save will be a no-op and nothing will be written to disk. Notably this method is called implicitly when updating actions on a FlowNode or when explicitly calling FlowNode.save.

We should throw an error in this case instead to make it easier for developers to detect and identify any issues. Generally speaking, to persist metadata for a possibly-completed build, you should use an Action on the Run that stores FlowNode.getId as needed.

TODO:

• Run PCT to check if this affects tests in significant plugins (Run PCT against workflow-cps PR #826 bom#2785)
• Add test (CpsFlowExecutionTest.flowNodesCantBeSavedAfterExecutionCompletes)

Testing done

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