glutton: replace stuck actors on resume/hibernate failure

[Chenyi Wang (chw120)]

Summary

Replace broken/stuck gluttonActor instances in place when resume() or hibernate() fails during benchmark iterations.

Problem

When SuspendActor fails (e.g., due to transient GCS ResourceExhausted errors during cold-bucket ramp-up), ate-api-server can leave the actor permanently stuck in ACTOR_STATE_SUSPENDING.

Previously, gluttonUser kept the broken gluttonActor in its u.actors slice for the remainder of the benchmark run:

• hibernate() ignored SuspendActor / PauseActor errors.
• Subsequent ResumeActor calls against that slot repeatedly failed with got: ACTOR_STATE_SUSPENDING for the rest of the test, reducing effective concurrency and generating continuous failure noise.

Solution

• Have hibernate(), pause(), and suspend() return bool indicating whether the operation succeeded.
• Add (*gluttonUser).replaceActor(ctx, broken) to delete the broken actor (AnyState: true) and replace its slot in u.actors with a freshly created actor (sb-<uuid>).
• Invoke user.replaceActor(ctx, actor) in iterate() when either actor.resume(ctx) or actor.hibernate(ctx) fails.
• Add unit test coverage in lifecycle_test.go (TestGluttonIterate_ReplacesActorOnResumeFailure).

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR