CVE-2026-54689: SearXNG MCP Server: Additional hardened-mode SSRF bypasses

mcp-searxng has a hardened-mode URL-reading feature intended to prevent web_url_read from reaching private or internal network resources.

PR #79 appears to address one SSRF class: hostnames that resolve to private or internal addresses under hardened mode. I tested PR #79 locally and confirmed that it blocks the DNS-resolves-to-loopback case.

However, several other hardened-mode SSRF bypasses still appear to remain:

1. Redirects from an allowed first-hop URL to a loopback/internal URL are followed without re-validating the redirect target.
2. 0.0.0.0 is not treated as an internal/special address.
3. IPv4-mapped IPv6 literals can bypass private-address checks after URL canonicalization.

With hardened mode enabled and private URLs not explicitly allowed, web_url_read was still able to fetch and return content from a local loopback sentinel service in all three cases.