feat(webapp,sdk,core): first-class pause and resume for concurrency limits

Concurrency limits can now be paused and resumed from the dashboard, the
API, and the SDK (`concurrencyLimits.pause()` /
`concurrencyLimits.resume()`), just like queues. A paused limit stops
every run holding it from being dequeued while keeping its configured
bounds, and resuming restores them.

Mono-RevId: 33cbe4dc731671f083779d05cedac03c3709a654