RPC agent crashes (unhandled process exit) on a single `null` input line

[dangzitou]

What happened?
A single line containing just null on stdin kills the whole RPC agent with an unhandled rejection, instead of returning an error response. Every other malformed line ([], 123, {}, "hi") is handled fine — only valid-JSON null is fatal:

$ printf 'null\n' | step --mode rpc
TypeError: Cannot read properties of null (reading 'id')
    at handleInputLine (...)
Node.js v22.23.2      <- process exits

One bad line (from a client that emits a half-written frame, or any upstream producer) tears down the running session/daemon.

Steps to reproduce

1. printf 'null\n' | step --mode rpc
2. Process exits non-zero with the TypeError above; it should stay up.

Root cause: packages/coding-agent/src/modes/rpc/rpc-mode.ts handleInputLine casts the parsed payload to RpcCommand and reads command.id in both handleCommand and the catch. On null the first read throws, then the catch reads command.id again and re-throws, escaping as an unhandled rejection.

Expected behavior
Emit an error response frame for that line and keep serving subsequent commands.

Version
0.84.4

I'd like to implement the fix myself.

[github-actions]

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[dangzitou]

Follow-up: the fix is ready and now rebased onto current main. The original PR #142 was auto-closed minutes after opening by the contributor gate, which has since been removed from main (#152).

Branch: dangzitou/Step-Code → fix/rpc-null-line-crash @ dee0e99 — a single commit, 11 lines in packages/coding-agent/src/modes/rpc/rpc-mode.ts. It guards null / non-object parsed payloads before the RpcCommand cast and emits the same error response frame other malformed input already produces, so a lone null line can no longer kill the RPC agent. Verified on current main with the existing RPC suites and the exact repro from this issue (before: uncaught TypeError, non-zero exit; after: error frame, exit 0 on stdin EOF). The bug is still present on main.

I can't land it myself right now:

• Reopening fix: reject null RPC command instead of crashing the agent #142 fails — its base commit is gone from main's rewritten history (GitHub: "Could not open the pull request").
• Opening a fresh PR from the same rebased branch fails with "dangzitou does not have the correct permissions to execute CreatePullRequest" (fork PRs from this account worked as recently as yesterday, e.g. fix: isolate sessions by header cwd #157).

If the branch looks right, could someone open the PR from it, or let me know how you'd like to proceed? Happy to adjust the message shape or squash it.