feat(electrum): optimize merkle proof validation with batching

[LagginTimes]

Replaces #1908, originally authored by @Keerthi421. Fixes #1891.

Description

This PR optimizes sync/full_scan performance by batching and caching key RPC calls to slash network round-trips and eliminate redundant work.

Key improvements:

• Gather all blockchain.transaction.get_merkle calls into a single batch_call request.
• Use batch_script_get_history instead of many individual script_get_history calls.
• Use batch_block_header to fetch all needed block headers in one call rather than repeatedly calling block_header.
• Introduce a cache of transaction anchors to skip re-validating already confirmed transactions.

Anchor Caching Performance Improvements

Results suggest a significant speed up with a warmed up cache. Tested on local Electrum server with:

$ cargo bench -p bdk_electrum --bench test_sync

Results before this PR (https://github.com/LagginTimes/bdk/tree/1957-master-branch):

sync_with_electrum      time:   [1.3702 s 1.3732 s 1.3852 s]

Results after this PR:

sync_with_electrum      time:   [851.31 ms 853.26 ms 856.23 ms]

Batch Call Performance Improvements

No persisted data was carried over between runs, so each test started with cold caches and measured only raw batching performance. Tested withexample_electrum out of https://github.com/LagginTimes/bdk/tree/example_electrum_timing with the following parameters:

$ example_electrum init "tr([62f3f3af/86'/1'/0']tpubDD4Kse29e47rSP5paSuNPhWnGMcdEDAuiG42LEd5yaRDN2CFApWiLTAzxQSLS7MpvxrpxvRJBVcjhVPRk7gec4iWfwvLrEhns1LA4h7i3c2/0/*)#cn4sudyq"
$ example_electrum scan tcp://signet-electrumx.wakiyamap.dev:50001

Results before this PR:

FULL_SCAN TIME: 8.145874476s

Results after this PR (using this PR's bdk_electrum_client.rs):

FULL_SCAN TIME: 2.594050112s

Changelog notice

• Add transaction anchor cache to prevent redundant network calls.
• Batch Merkle proof, script history, and header requests.

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

• [x] I've added tests for the new feature
• [x] I've added docs for the new feature

Bugfixes:

• [ ] This pull request breaks the existing API
• [x] I've added tests to reproduce the issue which are now passing
• [x] I'm linking the issue being fixed by this PR

[evanlinjin]

@LagginTimes could you provide the benchmark results in the PR description and compare it to results before the changes in this PR?

[notmandatory]

Based on above benchmark results it looks like this change is 1s faster on sync, is that due to a small test size? Do we expect it to make more of a difference with wallets with many addresses?

[evanlinjin]

@LagginTimes ~~can you provide the code you used to test with a remote electrum server (instead of the testenv)?~~

Edit: how about we just test with the example-cli with a pre-populated signet wallet?

I suggested writing benchmarks with the assumption that local io (against testenv) would be slower than allocating memory (collecting requests into vec before batch requesting), however that assumption seems incorrect.

[ValuedMammal]

These are my criterion results after benching this PR. 👍

sync_with_electrum      time:   [37.325 ms 38.220 ms 38.805 ms]                              
                        change: [-85.756% -85.556% -85.308%] (p = 0.00 < 0.05)
                        Performance has improved.

[ValuedMammal]

In 838c24762b8b03a34a64804b9b7a5e6626f03aad:

error: this file contains an unclosed delimiter
   --> crates/electrum/src/bdk_electrum_client.rs:724:3
    |
32  | impl<E: ElectrumApi> BdkElectrumClient<E> {
    |                                           - unclosed delimiter
...
504 |             for &(txid, height, hash) in chunk {
    |                                                - this delimiter might not be properly closed...
...
547 |         }
    |         - ...as it matches this but it has different indentation
...
724 | }