Without this call Gin's default trusts all X-Forwarded-For headers, letting
any caller rotate their effective IP and bypass per-IP rate limiting.
SetTrustedProxies(nil) forces c.ClientIP() to always return the real
TCP RemoteAddr.
Adds two regression tests: one documenting the pre-fix bypass, one
asserting the spoofed header is ignored after the fix.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>