Harden RTM template parsing to prevent DoS on malformed/unbounded tx data

[MoneroOcean]

Motivation

• Prevent availability attacks where a malicious or compromised RTM daemon can supply an unbounded or malformed transaction list that forces large allocations, synchronous parsing, extensive per-transaction logging, or uncaught exceptions in the single-threaded Node process.
• Restore bounds and defensive checks to the RTM merkle recomputation path so daemon-supplied template data cannot trigger excessive work or crash the pool.

Description

• Add a hard cap MAX_TEMPLATE_TRANSACTIONS = 5000 and validate the decoded transaction count, rejecting counts outside the allowed range.
• Validate transaction offsets against blob_in.length and throw on out-of-range offsets to avoid reading past the buffer.
• Wrap rtm.readTransaction in try/catch and validate the returned parsed.offset to ensure parsed sizes are sane before advancing.
• Guard public RTM entrypoints convertRtmBlob and constructNewRtmBlob with try/catch around update_merkle_root_hash so malformed daemon templates do not crash the process while preserving return shapes.

Testing

• Ran a Node syntax check with node -c index.js, which succeeded.
• No existing unit tests were modified or added as part of this change.

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Codex Task