Ingester stuck in LEAVING when its ring entry has 0 tokens #7899

Describe the bug
#7122 changed the LEAVING recovery check in Lifecycler.initRing to len(instanceDesc.Tokens) != 0 https://github.com/cortexproject/cortex/blame/master/pkg/ring/lifecycler.go#L805C55-L805C63, so ingesters that lost only some of their tokens can rejoin. An ingester whose entry is LEAVING with 0 tokens still skips that branch and keeps LEAVING with 0 tokens. autoJoin never runs, because it only runs from PENDING, and /ready keeps returning this instance owns no tokens. Restarting the pod or forgetting the instance doesn't get it out of this state.

Expected behavior
A LEAVING entry with 0 tokens should go back to PENDING so autoJoin can generate new tokens.

Proposed Fix

if instanceDesc.State == LEAVING {
    if len(instanceDesc.Tokens) == 0 {
        // No tokens to keep; restart the lifecycle so autoJoin generates new ones.
        instanceDesc.State = PENDING
    } else if i.autoJoinOnStartup {
        instanceDesc.State = i.getPreviousState()
    } else {
        instanceDesc.State = PENDING
    }
}