Implement blockchain.scripthash.get_mempool Electrum RPC method

[PeterXMR]

Refs #1241 — checks off the blockchain.scripthash.get_mempool item on the v1.6 protocol checklist.

Summary

blockchain.scripthash.get_mempool is required by Electrum protocol v1.6 and was not previously implemented in electrs.

Response shape matches blockchain.scripthash.get_history but filtered to mempool entries only — {tx_hash, height, fee} per entry, where height is 0 if all inputs are confirmed and -1 otherwise.

v1.6 ordering requirement: the spec mandates that get_mempool results "must be sorted ... same order as computing scripthash status". This implementation is a filter over the existing status history, so it shares the sort key with the status hash by construction — no separate ordering logic needed.

Test plan

• 1 new unit test (test_is_unconfirmed) in status::tests plus dispatch reuses the existing HistoryEntry JSON shape covered by test_txinfo_json
• cargo test --lib — 22/22 pass
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• Live tested against Bitcoin Core v31.0.0 on regtest. Set up a wallet, mined 101 blocks for mature coinbase, then sendtoaddress to a new address to produce a mempool entry:

$ printf '{"id":0,"method":"server.version","params":["test","1.4"]}
{"id":1,"method":"blockchain.scripthash.subscribe","params":["3c1f5806df22f16793a08cfb0e41070fabcf3c178d385072c15faec46b71a07c"]}
{"id":2,"method":"blockchain.scripthash.get_mempool","params":["3c1f5806df22f16793a08cfb0e41070fabcf3c178d385072c15faec46b71a07c"]}
{"id":3,"method":"blockchain.scripthash.get_history","params":["3c1f5806df22f16793a08cfb0e41070fabcf3c178d385072c15faec46b71a07c"]}
' | nc -w 4 127.0.0.1 60401

{"id":0,"jsonrpc":"2.0","result":["electrs/0.11.1","1.4"]}
{"id":1,"jsonrpc":"2.0","result":"c2eee7ea99189036be6b18f0c0e5d2ff2440977ff36717fcecf1f9449a6ff25a"}
{"id":2,"jsonrpc":"2.0","result":[{"fee":2820,"height":0,"tx_hash":"c8010e16520b033300d78ac385cdc1f59bed8ed0783d8836ec2009d0aa9c5583"}]}
{"id":3,"jsonrpc":"2.0","result":[{"fee":2820,"height":0,"tx_hash":"c8010e16520b033300d78ac385cdc1f59bed8ed0783d8836ec2009d0aa9c5583"}]}

After mining one block to confirm the tx, get_mempool correctly filters it out while get_history now reports it as confirmed:

{"id":2,"jsonrpc":"2.0","result":[]}
{"id":3,"jsonrpc":"2.0","result":[{"height":102,"tx_hash":"c8010e16520b033300d78ac385cdc1f59bed8ed0783d8836ec2009d0aa9c5583"}]}