Finalize recovered CPS executions with no threads

[AnantSingh1510]

Handle an unrecoverable Pipeline state where a CpsFlowExecution is restored with no CPS threads while its flow graph is still non-terminal.

When this happens, there is nothing for Pipeline to resume or interrupt, but the owning WorkflowRun can still remain marked as building. Treat an empty restored CpsThreadGroup on a non-complete execution as a program load failure, so the existing recovery path finalizes the run as FAILURE instead of leaving it stuck.

This also moves the done = true assignment in placeholder node creation until after the fallback nodes/actions are created, avoiding action persistence warnings on that path.

Fixes #1792

Testing done

Added PersistenceProblemsTest#inProgressWithStepEndHeadButNoCpsThreads.

The test creates the persisted state directly:

1. Run a simple Pipeline to completion.
2. Move the persisted flow head back to a StepEndNode.
3. Clear the WorkflowRun completion/result state.
4. Save an empty CpsThreadGroup to program.dat.
5. Simulate a hard Jenkins restart.
6. Verify the build is recovered as FAILURE.

Ran locally:

export JAVA_HOME=$(/opt/homebrew/bin/brew --prefix openjdk)/libexec/openjdk.jdk/Contents/Home
/opt/homebrew/bin/mvn -ntp -pl plugin -Dtest=PersistenceProblemsTest#inProgressWithStepEndHeadButNoCpsThreads test

Result:

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Related pull requests: none.

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