feat(acp-nats): add terminal_kill client handler

- Add client/terminal_kill handler for terminal/kill_command
- Wire ClientMethod::TerminalKill in dispatch
- Add terminal_kill subject and parsing
- Add jsonrpc_error_response for Result-returning handlers
- Add dispatch and subject tests

Signed-off-by: Yordis Prieto <yordis.prieto@gmail.com>

Author: yordis