[runtime] Fix prune state corner cases around checkpoints

[joeyutong]

Purpose of change

ActionExecutionOperator had two prune-state corner cases around checkpoints.

• prune bookkeeping used messageSequenceNumber, which tracks the latest started sequence, not the latest completed sequence. That means an in-flight sequence could be treated as prune-safe.

• the operator pruned action state as soon as a run completed. If the job later recovered from an earlier completed checkpoint, replay of the same input could no longer reuse the durable action state that had already been written.

Fix

This change makes pruning checkpoint-safe again.

• track a separate lastCompletedSequenceNumber keyed state
• snapshot prune metadata from lastCompletedSequenceNumber instead of messageSequenceNumber
• remove the eager prune path on run completion
• prune action state only after notifyCheckpointComplete

Tests

The updated tests cover:

• no pruning before checkpoint completion
• no pruning of sequences that are still in flight
• replay from an earlier checkpoint keeps durable state available
• action state cleanup happens after checkpoint completion

API

No

Documentation

• doc-needed
• doc-not-needed
• doc-included