Reject oversized tx_hash lists in construct_block_blob to mitigate DoS

[MoneroOcean]

Motivation

• Prevent a remote daemon from triggering unbounded stack allocation in the merkle tree_hash path by rejecting overly large merged-mining templates before construct_parent_block is invoked.

Description

• Add a conservative hard limit MAX_TX_HASHES_FOR_PARENT_BLOCK = 16384 to limit how many tx_hashes are accepted for BLOB_TYPE_FORKNOTE2.
• Validate b.tx_hashes.size() in construct_block_blob and return an error if the template exceeds the limit, avoiding the downstream call chain that leads to stack alloca in tree_hash.

Testing

• Ran the package test command npm test, which failed in this environment due to a network/registry 403 Forbidden error when fetching dependencies (specifically bech32), so automated tests could not complete.

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Codex Task