## What
Add `CountWorkers` RPC to count workers matching a query filter without
retrieving full worker details.
## Why
The UI needs to display a worker count in places where listing isn't
necessary. A dedicated count API follows the existing pattern
(`CountWorkflowExecutions`, `CountSchedules`).
## How did you test it?
- [x] new unit tests: count all, count with query filter, count with no
matches, invalid query error
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
c0d871bc9a