feat(electrum): return JSON-RPC 2.0 error objects (port of mempool/electrs#103)

[EddieHouston]

Resolves #221 by porting mempool/electrs#103 onto new-index and extending it with
error categorization. Supersedes #220 — this includes that unknown-method fix in
error-object form.

What changes on the wire

All Electrum RPC error replies are now JSON-RPC 2.0 error objects instead of bare
strings, and malformed input gets a reply instead of a dropped connection:

Case	Before	After
Unknown method	connection dropped	{"code": -32601, "message": "unknown method <name>"}
Bad/missing params	"error": "missing height"	{"code": -32602, "message": "missing height"}
History too large (TooPopular)	bare string	{"code": 1, ...} (ElectrumX-style bad request)
bitcoind RPC error	bare string	{"code": 2, ...} (ElectrumX-style daemon error)
Other handler errors	bare string	{"code": -32603, ...}
Unparseable JSON	connection dropped	{"code": -32700, "message": "parse error"}, "id": null
Valid JSON, invalid request shape	connection dropped	{"code": -32600, "message": "invalid request"}, "id" echoed or null
Unknown method inside a batch	whole batch + connection dropped	that entry errors, others answered

Success replies are unchanged. Oversized batches, invalid UTF-8, and TLS bytes on the
plaintext port still close the connection (peers not speaking the protocol).

Implementation

• JsonRpcV2Error enum and json_rpc_error() helper ported from Fix JSON-RPC v2 errors mempool/electrs#103
  (mempool/electrs@277b05b), with two deviations:
  the reply always includes "id" (null when undetectable, per spec — mempool omits
  it) and error messages don't echo request params back.
• The envelope handling (-32700/-32600) is re-implemented rather than picked,
  since mempool's hunk targets their refactored connection layer; handle_value now
  returns Value and keeps this repo's RPC event logging.
• New ErrorKind::InvalidParams carries param-validation failures from the
  *_from_value helpers to the dispatcher, where jsonrpc_code() maps error kinds to
  codes (InvalidParams → -32602, TooPopular → 1, RpcError (daemon) → 2,
  everything else → -32603). A few call sites stop wrapping param errors with
  chain_err, which previously erased the kind.

Testing

New integration test test_electrum_jsonrpc_errors covers: -32601 with the connection
left open, -32602 on a missing param, -32600 on a method-less request (id echoed),
-32700 on unparseable JSON (null id), a batch where an unknown method no longer kills
the other entries, and that the connection survives all of the above.

cargo test passes (lib + electrum + REST integration tests); cargo check is clean
on --features liquid and --features electrum-discovery.