Add RandomRPC node selection mode

[poopoothegorilla]

Summary

• Adds a new RandomRPC node selection mode that picks a random healthy RPC on every SelectRPC call, bypassing the sticky activeNode cache
• Preserves application-layer subscriptions during RPC rotation by not calling UnsubscribeAllExceptAliveLoop when switching nodes
• Disables lease-based rotation for this mode (same as existing RoundRobin)

Motivation

All existing selection modes are sticky — selectNode() returns the cached active node as long as it's alive. This new mode distributes RPC calls across healthy nodes without tearing down subscriptions on rotation.

Ref: PLEX-2537

Changes

• node_selector.go: New NodeSelectionModeRandomRPC constant + factory registration
• node_selector_random_rpc.go: Selector using math/rand/v2 to pick a random alive node
• multi_node.go: selectRandomRPCNode() path that skips activeNode caching and subscription teardown; lease check disabled for RandomRPC
• Tests for selector behavior and MultiNode integration (non-sticky, no-unsub, lease-disable, error cases)

[github-actions]

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[dhaidashenko]

Approving. Once merged, we should soak it on staging