fix(auth): drop stale Role field from Session test literal

Session.Role was removed from the base branch; the constant-time
mismatch test in service_test.go still referenced it, breaking
go vet. Remove the field from the struct literal; the test logic
is unaffected (it verifies token-hash rejection, not role checks).

Author: cristim

internal/auth/service_test.go

@@ -206,7 +206,6 @@ func TestService_ValidateSession(t *testing.T) {
 			Token:     "completely-different-hash",
 			UserID:    "user-999",
 			Email:     "mismatch@example.com",
-			Role:      RoleUser,
 			ExpiresAt: time.Now().Add(time.Hour),
 		}