fix(proxy/freedom): prevent crash when passthrough is "origin" or "srcip"

In certain routing scenarios, outbound detour tags like "origin" or "srcip" may be assigned 
to freedom handlers without proper initialization, causing nil pointer dereference during Dial().

This patch adds guard checks to ensure the outbound handler is valid before dialing, and improves 
fallback logic for local address passthrough (e.g., 127.0.0.1) to avoid kernel crashes.

Also ensures `sendThrough` behavior gracefully degrades if loopback or empty DNS response is encountered.