Propagate full electrum pool errors

[SihyeonJeon]

Fixes #702.

Summary

• preserve the full MultiError context when electrum-pool converts all failed attempts into the public electrum_client::Error
• avoid dropping every failed server except the last one in call / call_async
• add coverage proving the public error message includes both failing Electrum server URLs

Why

ElectrumBalancer::call_sync already records every failed attempt in MultiError, but the compatibility conversion into electrum_client::Error previously returned only the last error. That made the normal call and call_async paths lose the useful per-server failure details.

The new conversion keeps the existing public return type while wrapping the full MultiError display in the final I/O error.

Note on bitcoindevkit/rust-electrum-client#186: this is complementary to the upstream retry aggregation fix. If upstream returns a better AllAttemptsErrored, this change still keeps the electrum-pool aggregate from collapsing to a single server failure when exposed through call / call_async.

Tests

• rustfmt --check --edition 2024 electrum-pool/src/lib.rs
• cargo test -p electrum-pool test_call_preserves_all_errors_in_public_error
• cargo test -p electrum-pool test_call_all_clients_fail
• cargo test -p electrum-pool
• cargo check -p electrum-pool
• cargo clippy -p electrum-pool --all-targets -- -D warnings
• git diff --check

Not run locally: full workspace checks. A broad cargo test --workspace --no-run attempt failed in the local monero-sys setup because the expected Monero C++ target file was absent in this clone.

AI assistance: OpenAI Codex helped prepare the initial patch and this PR draft. The diff and checks above were reviewed before submission.