Honor zero-hour PaperBench checkpoint selection

[sylvesterkaczmarek]

Summary

• preserve target_duration_hr=0 as a valid PaperBench checkpoint target
• keep None as the sentinel for selecting the latest available checkpoint
• add regression coverage showing that a zero-hour target selects the earliest submission

PBTask._select_checkpoint() currently uses a truthiness check when choosing the duration passed to get_file_at_duration(). Because 0 is falsey, an explicit target_duration_hr=0 is replaced with 10000, which selects the latest checkpoint instead of the checkpoint at the start of the run.

The configuration type and get_file_at_duration() both support zero as a valid non-negative duration. This change switches the fallback check to is not None, so zero keeps its documented meaning while the existing None behavior remains unchanged.