Reject embedded NUL bytes in module import paths

jq accepts embedded NUL bytes at the language level but resolves
module import paths through NUL-terminated C strings, so the path
validated by policy or audit code could differ from the on-disk
path jq actually opens. Pass jv through gen_import so the AST
preserves the original bytes, and reject embedded NULs in
validate_relpath.

Fixes CVE-2026-43895.