fix: distinguish dropped txs from mempool txs in esplora confirmations

[matthewjablack]

Summary

Root cause: get_transaction_confirmations in esplora.rs returned Ok(0) for both mempool txs and dropped/never-broadcast txs. With NB_CONFIRMATIONS=0, the manager's 0 >= 0 check always passed, promoting contracts through states even when their funding tx was never mined.

Changes

ddk/src/chain/esplora.rs — the core fix:

• When the tx is unconfirmed (confirmed: false), call get_tx to check if it's actually in the mempool
• If it exists in mempool: return Ok(0) (existing behavior)
• If the tx is gone (dropped/never-broadcast): return Err(TransactionNotFound) instead of Ok(0)
• Consumers can now distinguish mempool from dropped
• Moved get_height() call inside the confirmed branch (avoids wasted call when unconfirmed)

ddk-manager/src/manager.rs — defense-in-depth + recovery:

• close_contract / check_and_broadcast_refund / check_refund: use unwrap_or(0) since the tx may not exist yet (pre-broadcast)
• check_confirmed_contract pending_close_txs: skip dropped txs via Err(_) => continue
• check_for_spliced_contract: skip pre-close only when the splice fund tx is dropped (Err), not when it's in the mempool (Ok(0))
• check_preclosed_contract: on confirmation error for splice closes (no attestations), revert to Confirmed instead of propagating
• recover_incorrectly_closed_contracts: new periodic check that finds splice-closed contracts whose closing tx is gone and reverts them to Confirmed

Context

On testnet4 with NB_CONFIRMATIONS=0, a splice fund tx entered the mempool and ddk walked the parent contract Confirmed → PreClosed → Closed in a single periodic_check cycle. The splice tx was later dropped from mempool, but the contract was already Closed. Subsequent splice attempts failed at get_signature_for_dlc_input because it only accepts Confirmed contracts.