Remove zombie node logic and add out-of-sync fallback selection

[poopoothegorilla]

Description

Remove "zombie" node logic that incorrectly kept unhealthy nodes alive when they were the last node in the pool, which caused missed state transitions and silent metric gaps (PLEX-2538)
Add fallback node selection that uses out-of-sync nodes when no alive nodes are available, with automatic upgrade back to alive nodes when they recover

Problem

When two nodes (A and B) exist and node A is in FinalizedBlockOutOfSync state, LatestChainInfo() does not count it as alive (since State() returns nodeStateFinalizedBlockOutOfSync). If node B then fails health checks, the l < 2 guard in the alive loop mistakenly treats B as the "last alive node" and keeps it in a zombie state instead of transitioning it to Unreachable or OutOfSync. These zombie nodes are invisible to metrics, preventing proper alerting.

Changes

node_lifecycle.go: Remove all zombie node guards from aliveLoop (4 locations) and outOfSyncLoop (1 location). Nodes now always transition to their correct state regardless of pool size. Remove zombieNodeCheckInterval(), msgCannotDisable, and msgDegradedState.
multi_node.go: Add selectOutOfSyncNode() fallback that picks the best out-of-sync node by highest block number. Update awaitNodeSelection() to fall back to out-of-sync nodes when no alive nodes exist. Update selectNode() to upgrade from an out-of-sync active node to an alive one when available. Update checkLease() to handle nil selector results with out-of-sync fallback.

Tests: Update 5 zombie behavior tests to verify correct state transitions. Add 6 new tests for out-of-sync fallback selection, best-node picking, and alive-node upgrade.

Test plan

[x] All existing multinode tests pass (50/50)
[x] Zombie tests updated to verify nodes transition to correct unhealthy state
[x] New tests cover: fallback to OutOfSync node, fallback to FinalizedBlockOutOfSync node, best-node selection by block height, keeping out-of-sync active when no alive available, upgrading from out-of-sync to alive

[github-actions]

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[dhaidashenko]

why remove this tests?

[poopoothegorilla]

Addressed — restored both zombie behavior tests: 'forcibly keeps it alive' (with poolInfo mock returning 0 live nodes excluding caller) and 'but is a proxy, transitions to unreachable'. Updated mocks to use LatestChainInfo(mock.Anything).