fix(electrum): check batch response lengths before pairing

fix(electrum): check batch response lengths before pairing

`BdkElectrumClient` paired every batch request with its response via
`zip` without checking that the response had one entry per request. A
short response (possible with any non-stock `ElectrumApi` implementation)
silently dropped trailing scripts, txids and anchors while `sync` still
returned `Ok`, and a short `batch_block_header` response left
`height_to_hash` incomplete so `height_to_hash[&h]` panicked.

Add `check_batch_len` and call it after each of the five batch calls
(`batch_script_get_history` x3, `batch_block_header`,
`batch_transaction_get_merkle`), returning `Error::Message` on any
length mismatch, in line with the other server-misbehaviour checks in
this crate.

Fixes #2305