Summary

Reject unrecognized SystemTransaction variants with an explicit error instead of silently labeling them as "unknown" for metrics, closing an observability gap identified by the Least Authority audit.

🎫 Ticket 📐 Engineering 🧪 Test Plan

Motivation

A security audit (Least Authority, Issue AC) identified that get_system_tx_type in the ledger host function used a wildcard match arm (_ => "unknown") for unrecognized SystemTransaction variants. While the binary deserialization layer already prevents truly malformed transactions from reaching this code, the silent labeling creates an observability gap: if a newer midnight-ledger introduces a new variant, an older node build would quietly process it without any warning or error, making version mismatches invisible to operators.

Changes

• get_system_tx_type — Changed return type from &'static str to Result<&'static str, LedgerApiError>. The wildcard arm now logs an error and returns Err(SystemTransactionError::UnknownError) (code 204). Extracted from impl Bridge<S, D> to a standalone free function to enable direct unit testing (the generic bounds on the impl block prevented calling it from test modules).
• apply_system_transaction — Propagates the new Result via ?, rejecting unknown variants before processing.
• Tests — 7 unit tests covering all constructible SystemTransaction variants (7/8 known variants; OverwriteParameters requires complex LedgerParameters construction). The wildcard path is untestable by design due to #[non_exhaustive].
• Change file — Added audit-handle-unsupported-system-transaction-type.md.

📌 Submission Checklist

• Changes are backward-compatible (or flagged if breaking)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason: N/A
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• No new todos introduced

🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other
• N/A

🗹 TODO before merging

• Ready for review