fix(electrum): stop logging full response payload on client disconnect

[EddieHouston]

Problem

A client that disconnects mid-response (broken pipe / connection reset) caused the entire JSON-RPC reply to be logged at ERROR. For large replies — e.g. a blockchain.scripthash.get_history result for a very popular address — this dumped a multi-megabyte line to stderr, which Cloud Logging then split across many ERROR entries.

Root cause chain in src/electrum/server.rs:

1. send_values() built its error context with format!("failed to send {}", value), embedding the entire reply Value into the error string.
2. That error propagated up to run() and was logged via error!(... e.display_chain() ...).
3. stderrlog writes to stderr, which GKE/Cloud Logging tags as ERROR severity by default.

Fix

• Truncate the send-failure context to a byte count instead of the full payload:

  .chain_err(|| format!("failed to send response ({} bytes)", line.len()))?;

• Add an is_disconnect() helper that walks the error source() chain for BrokenPipe | ConnectionReset | ConnectionAborted | UnexpectedEof.
• In run(), route client disconnects to debug! (expected, not actionable) while genuine failures still log at error!.

Result

No more multi-megabyte ERROR spam. A client disconnecting mid-response now produces a single concise debug! line; real write/protocol failures remain at error! without the payload.