fix: Propagate context to external signer in DataPoster signer

[vtjl10]

• Summary: Replace context.TODO() with the constructor-provided ctx in bind.TransactOpts.Signer (file arbnode/dataposter/data_poster.go) so sign requests respect cancellation and deadlines.
• Behavior: Signing now honors the provided Context; functional behavior remains unchanged.
• Rationale: Avoids potential hangs during shutdown or network stalls; improves robustness and resource handling.