fix(electrum): return JSON-RPC error for unknown methods instead of dropping the connection

[EddieHouston]

Resolves #219

Problem

Any method without a dispatch arm in handle_command — a typo, an unimplemented
Electrum method (blockchain.scripthash.get_mempool,
blockchain.transaction.get_tsc_merkle), or blockchain.scripthash.get_balance on
Liquid builds (where the arm is compiled out via #[cfg(not(feature = "liquid"))]) —
causes the server to close the TCP connection without writing any JSON-RPC response.
Clients cannot distinguish an intentionally unsupported method from an outage, and
retrying clients turn one cheap error reply into repeated TCP+TLS reconnects.

Inside a batch request, a single unknown method kills the whole batch and the
connection; the remaining requests are never answered.

Cause

The catch-all arm in handle_command:

&_ => bail!("unknown method {} {:?}", method, params),

bail! returns Err from the whole function, bypassing the error-to-JSON conversion
a few lines below — that conversion only sees errors that dispatched handlers assign
into result. The unknown-method error instead propagates through the ? in
handle_value and handle_replies, and run() logs "connection handling failed"
and shuts the socket down.

Fix

Produce the error as a value inside result, so it flows through the existing
conversion and is returned as a normal JSON-RPC error reply:

&_ => Err(format!("unknown method {}", method).into()),

• The connection stays open, matching other handler errors in electrs and other
  Electrum server implementations (ElectrumX, Fulcrum).
• params are deliberately omitted from the message so client-controlled input is
  not echoed back; only the method name is included.
• Batches now answer their remaining requests when one entry has an unknown method.

Prior art

Both related projects have already fixed this. mempool/electrs (a fork of this
codebase) replaced the catch-all in mempool/electrs#103 (merged 2024-11-14) to return
a JSON-RPC error object with code -32601 and keep the connection open;
romanz/electrs (the v0.9 rewrite) returns {"code": -32601, "message": "method not found"} via its typed RpcError enum. This PR keeps the existing bare-string error
shape for minimal diff; moving to JSON-RPC 2.0 error objects across the board is left
to a follow-up.

Out of scope

Malformed input still drops the connection, unchanged: invalid JSON, requests
without a string method / array params / id, oversized batches, invalid UTF-8,
and TLS bytes on the plaintext port. The first two arguably deserve error replies as
well (with "id": null per JSON-RPC), but that is a larger change than this fix.

Testing

Against a local instance:

echo '{"jsonrpc":"2.0","id":1,"method":"blockchain.scripthash.get_mempool","params":["e573ed3c89a107e4c5b67cebc6a6cadb2faa40fb02a7c5092d9b4d4f118a3efe"]}' | nc localhost 50001

Before: connection closes with no payload.
After: {"error":"unknown method blockchain.scripthash.get_mempool","id":1,"jsonrpc":"2.0"}
and the connection remains open. A batch mixing server.ping with an unknown method
now returns results for both entries instead of dropping the connection.

Full cargo test (lib + integration: 4 electrum, 22 REST) passes.