Carry operator cache-reuse status to the frontend and handle cached-region statistics

[Xiao-zhen-Liu]

Task Summary

Part of #5881 (design: #5880).

Add the operator state that marks a result as reused from the cache, and the small statistics changes for a region that did not run. Additive: nothing produces the new state yet (the scheduler task #5884 does), so it changes no behavior on its own.

Scope

• A COMPLETED_FROM_CACHE value in the WorkflowAggregatedState enum, with the matching string and status-code conversions, treated as a terminal state in state aggregation.
• Reported statistics treat a negative count as unknown rather than summing it. A region that did not run has no real input or output counts and reports them as unknown; this keeps totals correct for normal runs.
• A guard so an operator that finishes instantly (a skipped region has zero workers) still triggers the final result update.

Why this is safe. The new enum value is added at the end, so existing values keep their numbers, and nothing emits it yet. The statistics change only affects negative counts, which a normal worker never reports.

Notes. The enum is in a .proto file; generated protobuf code is not committed, so the change ships the .proto edit only.

Depends on. None.

Out of scope. Producing the state at runtime (#5884) and any UI for it (#5886).

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other