Test that IPv4-mapped login clients are counted apart (closes #66)

Two IPv4 clients that the trusted proxy forwards in IPv4-mapped form must
not share one login count. Fails: both fall in the same /64.

Model: opus-5-5

internal/server/login_rate_limit_internal_test.go

@@ -260,3 +260,21 @@ func TestLoginRateLimitCountsIPv6ClientsByPrefix(t *testing.T) {
 			rec.Code, http.StatusOK)
 	}
 }
+
+// TestLoginRateLimitCountsIPv4MappedClientsSeparately verifies an IPv4
+// client that the proxy forwards in IPv4-mapped IPv6 form (::ffff:a.b.c.d)
+// is counted by its IPv4 address, not by the /64 that every such address
+// shares, so two of them behind the proxy are counted separately.
+func TestLoginRateLimitCountsIPv4MappedClientsSeparately(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	tripLoginRateLimit(t, s, proxyPeer, "::ffff:"+firstForwarded)
+
+	rec := postLogin(t, s, proxyPeer, "::ffff:"+secondForwarded, wrongKey)
+	if rec.Code != http.StatusOK {
+		t.Errorf("failed login from a second IPv4-mapped client "+
+			"status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}