fix(email): assert the SMTP rejection structurally, not on textproto's text

Go 1.26 changed net/textproto.Error.Error() to quote the message
("%03d %q"), so the rejection renders as `501 "5.5.4 Rejected!"` and the
substring match on `501 5.5.4 Rejected!` no longer holds. TestEmailRejected
has been failing on this toolchain ever since.

Unwrap to the *textproto.Error and assert Code and Msg instead. That is
stronger than the substring it replaces — it pins the numeric code and the
exact message rather than a fragment of a formatted string — and it does
not depend on how the stdlib renders the error.

The second failure in the same test was a cascade: the require above
aborted the test before srv.Shutdown ran, so the cleanup's ErrServerClosed
check saw a nil error from its own first shutdown.

go test ./...: 130 ok / 1 FAIL -> 131 ok / 0 FAIL.

Co-authored-by: Hanzo Dev <[email protected]>

pkg/alertmanager/alertmanagernotify/email/email_test.go

@@ -11,6 +11,7 @@ import (
 	"log/slog"
 	"net"
 	"net/http"
+	"net/textproto"
 	"net/url"
 	"os"
 	"strconv"
@@ -719,7 +720,13 @@ func TestEmailRejected(t *testing.T) {
 
 	// Send the alert to mock SMTP server.
 	retry, err := e.Notify(context.Background(), firingAlert)
-	require.ErrorContains(t, err, "501 5.5.4 Rejected!")
+	// net/smtp surfaces the rejection as *textproto.Error. Assert on its fields rather
+	// than on the rendered text: Go 1.26 changed textproto.Error.Error to quote the
+	// message ("%03d %q"), so matching the formatted string is toolchain-dependent.
+	var protoErr *textproto.Error
+	require.ErrorAs(t, err, &protoErr)
+	require.Equal(t, 501, protoErr.Code)
+	require.Equal(t, "5.5.4 Rejected!", protoErr.Msg)
 	require.True(t, retry)
 	require.NoError(t, srv.Shutdown(ctx))