Add per-workflow duplicate start suppression option

[vince-chang-airbnb]

Summary

• Add a default-off WorkflowOptions.createExistingWorkflowIsNoop option for workflows that must suppress duplicate scheduling.
• Propagate the option through WorkflowFactory and RunRequest so SkipperEngine can return an existing workflow instance without scheduling another execution.
• Preserve the existing process-wide feature gate and the explicit RETRIES_EXHAUSTED resume path.
• Preserve existing WorkflowOptions and RunRequest constructor signatures while defaulting the new option to false.

For a non-terminal workflow, this suppresses another execution and returns the current persisted instance. A later invocation after completion receives the persisted terminal result.

Testing

• ./gradlew :test --tests com.airbnb.skipper.WorkflowOptionsTest --tests com.airbnb.skipper.WorkflowFactoryTest --tests com.airbnb.skipper.internal.SkipperEngineTest
• ./gradlew build