The skip_existing_in behavior of QuantumGraphBuilder was previously
only covered through test_separable_pipeline_executor.py, where
SeparablePipelineExecutor drives AllDimensionsQuantumGraphBuilder.
No tests exercised the builder directly at the unit level.

…quanta

Daytime AP runs against data produced by Prompt Processing, which does
not retain all intermediate outputs.  With --skip-existing-in, tasks
whose metadata exists are skipped even when their outputs are absent.
When a downstream task needs to run, it may not see some inputs and
is dropped as no work found.

retained_dataset_types provides dataset types expected to exist in
skip_existing_in. The non-retained types trigger backward unskipping
of the ancestor quanta needed to regenerate them.