Replace context.TODO with cancellable, timed context for external signer in DataPoster

[maradini77]

• Summary: Propagate cancellation and add a 30s timeout to external signer RPC calls instead of using context.TODO.
• What/Why:
  • Prevents indefinite hangs during shutdown or signer stalls.
  • Ensures RPC calls respect upstream cancellation and deadlines.
• Changes:
  • arbnode/dataposter/data_poster.go: use context.WithTimeout(ctx, 30*time.Second) in TransactOpts.Signer.

[eljobe]

I don't believe this change either behaves as described in the PR description, or is safe and compatible with our codebase.

The DataPoster is a long-lived object, the lifecycle of the context object with which a new instance was created might be quite different than the lifecycle of the external signer being created for it to use when posting data to the parent chain.

These are not per-request contexts. They are essentially process-scoped contexts.

Closing.