[Feature]: Workflow Insight scheduler drops records across concurrent invocations

### What would you like?

Description

ExportScheduler uses one global pending slot. Records from different execution ARNs can overwrite each other, even though they are unrelated snapshots.

### Possible Implementation

- Records from different execution ARNs must never replace each other.
- Terminal records must always reach the exporter before drain completes.
- Burst coalescing must be scoped and documented.

### Is this a breaking change?

No

### Does this require an RFC?

No

### Additional Context

_No response_