Investigate null byte exclusion

[Liam-DeVoe]

We have this code:

    if schema_type == "string":
        # Exclude null bytes due to reflect-cpp truncation bug:
        # https://github.com/getml/reflect-cpp/issues/559
        # Exclude surrogates (Cs category) as they're invalid in UTF-8/JSON
        return st.text(
            alphabet=st.characters(
                blacklist_characters="\x00",
                blacklist_categories=("Cs",),  # type: ignore[arg-type]
            ),
            min_size=schema.get("min_size", 0),
            max_size=schema.get("max_size"),
        )

This should not be a server-side restriction, and should be a library-side restriction imposed by hegel-cpp over the wire instead.