Add basic libFuzzer target for protocol parsing

[digiscrypt]

Hi,

This adds a basic libFuzzer target for the C++ protocol layer.

It exercises BinaryProtocol and CompactProtocol by feeding arbitrary input through message and struct parsing paths, with simple limits to avoid runaway recursion and very large allocations.

The goal is to cover core deserialization logic and make OSS-Fuzz integration easier.

Let me know if you want anything changed.

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