test(11-07): cover flare VAPID, allowlist, statuses and config

- TestVAPIDHeader (origin rules, exp, subject), TestVAPIDKeys,
  TestSendAllowlist (T-11-22 host table), TestSendStatuses (2xx, 404/410,
  StatusError without body, disabled, host-only transport errors),
  TestFlareConfig, TestAgo, TestEncryptRejects (coverage 90.0%)

modules/flare/encrypt_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"crypto/ecdh"
 	"encoding/base64"
+	"errors"
 	"strings"
 	"testing"
 )
@@ -110,3 +111,35 @@ func TestRFC8291AppendixA(t *testing.T) {
 		t.Fatalf("3993-byte payload: %v", err)
 	}
 }
+
+// TestEncryptRejects covers the RFC 8291 input checks: a payload over
+// 3993 bytes, a p256dh that is not a 65-byte P-256 point, an auth secret
+// that is not 16 bytes and non-base64url input are refused.
+func TestEncryptRejects(t *testing.T) {
+	sub := newTestSubscriber(t, "https://fcm.googleapis.com/fcm/send/x").sub
+	if _, err := Encrypt(make([]byte, MaxPayloadSize+1), sub); !errors.Is(err, ErrPayloadTooLarge) {
+		t.Fatalf("oversized payload: %v", err)
+	}
+	if _, err := Encrypt(make([]byte, MaxPayloadSize), sub); err != nil {
+		t.Fatalf("payload at the limit: %v", err)
+	}
+	short := sub
+	short.P256dh = base64.RawURLEncoding.EncodeToString(make([]byte, 33))
+	offCurve := sub
+	offCurve.P256dh = base64.RawURLEncoding.EncodeToString(append([]byte{4}, make([]byte, 64)...))
+	badAuth := sub
+	badAuth.Auth = base64.RawURLEncoding.EncodeToString(make([]byte, 8))
+	notB64 := sub
+	notB64.Auth = "***"
+	for name, s := range map[string]Subscription{"short_key": short, "off_curve": offCurve, "short_auth": badAuth, "auth_not_base64": notB64} {
+		if _, err := Encrypt([]byte("x"), s); err == nil {
+			t.Errorf("%s accepted", name)
+		}
+	}
+	if b, err := decodeBase64URL("YWJj"); err != nil || string(b) != "abc" {
+		t.Fatalf("decode unpadded: %q %v", b, err)
+	}
+	if b, err := decodeBase64URL("YQ=="); err != nil || string(b) != "a" {
+		t.Fatalf("decode padded: %q %v", b, err)
+	}
+}