mempool startup can consume tip action before close-to-tip gate opens

[ValarDragon]

Problem

The mempool startup gate currently requires both:

• SyncStatus::is_close_to_tip() to be true, and
• a fresh TipAction from ChainTipChange::last_tip_change() so the mempool can initialize last_seen_tip_hash.

But poll_ready() calls last_tip_change() before checking whether startup is allowed. last_tip_change() is consuming: once it returns a tip action, it records that block hash as last_change_hash, so the same action is not returned again.

That creates a startup latch edge case:

1. A disabled mempool polls and consumes a tip action.
2. Zebra is still considered far from tip, so update_state() matches (false, false, _) and leaves the mempool disabled.
3. Later, RecentSyncLengths / SyncStatus independently changes so is_close_to_tip() becomes true.
4. No new tip action exists, so last_tip_change() returns None.
5. update_state() matches (true, false, None) and still refuses to enable.

The mempool then remains disabled until another chain tip change arrives.

Desired behavior

Startup should not require a fresh, unconsumed TipAction at the exact moment the close-to-tip gate opens.

Possible fixes:

• initialize the disabled mempool directly from LatestChainTip once is_caught_up_to_start() is true, or
• preserve a consumed tip action until activation succeeds.

This is separate from active-mempool reset behavior. Resetting/clearing the mempool on TipAction::Reset is fine; the issue is only that disabled startup can consume its one available tip action too early.

Code paths

• zebrad/src/components/mempool.rs: poll_ready() calls chain_tip_change.last_tip_change() before update_state().
• zebrad/src/components/mempool.rs: update_state() refuses (true, false, None) because there is no tip action to seed last_seen_tip_hash.
• zebra-state/src/service/chain_tip.rs: ChainTipChange::last_tip_change() updates last_change_hash when it returns an action.