GHSA-m8j6-rc5x-wv36: nono-py's policy JSON accepts unknown security fields

nono-py policy handling could fail open in two ways. First, resolving a policy-derived ProxyConfig did not automatically enforce CapabilitySet.proxy_only, allowing sandboxed children to bypass a resolved domain allowlist by using direct network access. Second, policy JSON accepted unknown security-sensitive fields, so misspelled or unsupported restrictions could be silently ignored.