Coalesce empty checkpoints to reduce API calls in high-concurrency map/wait scenarios

[AdityaAudi]

Summary

Implements the empty-checkpoint coalescing optimization described in #325.

When many concurrent map/parallel branches resume from a wait operation simultaneously, each branch's resubmitter enqueues an empty checkpoint (no OperationUpdate). Previously, these counted toward the 250-operation batch limit, causing 300 concurrent branches to produce 2 API calls (250 + 50) instead of 1.

• Tracks effective_operation_count separately from len(batch)
• The first empty checkpoint in a batch counts as 1 effective operation
• All subsequent empty checkpoints in the same batch are coalesced in without incrementing the effective count
• Applied consistently across all three collection phases: overflow queue drain, first-op blocking wait, and time-window loop
• Updated debug logging to clearly show real ops sent vs empties excluded