fix: reject pending checkpoints in decoded checkpoint artifacts

[matthias-wright]

Addresses #253

Changes:

• Rejects checkpoint artifacts that decode to consensus state with a non-None pending_checkpoint. This closes the checkpoint recovery path that could carry staged checkpoint data into finalized artifacts.
• Adds a regression test.

[evonide]

This is a good improvement but seems narrower than #253. It rejects finalized checkpoint artifacts that contain pending_checkpoint which closes the main checkpoint import path.

However, the underlying invariant was that a Checkpoint should not be trusted with digest != sha256(data) and raw Checkpoint::read_cfg / ConsensusState::read_cfg still appear to accept an embedded pending checkpoint without validating that binding. Could we either validate the checkpoint data/digest when decoding/reading Checkpoint or recompute/check the pending checkpoint digest right before the finalizer consumes it?

[matthias-wright]

Update(20d3847);

• Validate checkpoint digest against data on decode