reject invalid message length in MessageSerializer.readMessage - #1361

What's Changed

MessageSerializer.readMessage reads the 4-byte little-endian length prefix and hands it straight to ByteBuffer.allocate(messageLength) without checking it. When a stream ends right after a continuation token (0xFFFFFFFF) messageLength is left at -1, and a malformed prefix can carry any other negative value. Both reach ByteBuffer.allocate with a negative argument and throw IllegalArgumentException, which escapes the IOException / end-of-stream contract that ArrowStreamReader, ArrowFileReader and MessageChannelReader rely on when reading untrusted input.

Treat a continuation token with no following length as end-of-stream (return null, matching how a short first read is already handled) and reject a negative length with an IOException before the allocation. The guard sits in readMessage because every IPC reader funnels through it.

AI assistance was used to prepare this change; I reviewed the diff and test results before submitting.