fix(horizon): respect minimum delegation requirement in slash test

The testSlash_RoundDown_TokensThawing_Delegation test was failing because
it could generate scenarios where undelegating would leave less than the
minimum delegation amount (1 ether) in the pool, which violates the
protocol's minimum delegation constraint.

Added assumption to ensure undelegation either removes all tokens or
leaves at least MIN_DELEGATION in the pool, making the test compliant
with the protocol's validation rules introduced in commit 91cda56.

Author: RembrandtK

packages/horizon/test/unit/staking/slash/slash.t.sol

@@ -172,6 +172,8 @@ contract HorizonStakingSlashTest is HorizonStakingTest {
         vm.assume(delegationTokensToSlash <= delegationTokens);
         vm.assume(delegationTokensToUndelegate <= delegationTokens);
         vm.assume(delegationTokensToUndelegate > 0);
+        // Ensure that after undelegating, either we undelegate everything or leave at least MIN_DELEGATION
+        vm.assume(delegationTokensToUndelegate == delegationTokens || delegationTokens - delegationTokensToUndelegate >= MIN_DELEGATION);
 
         resetPrank(users.delegator);
         _delegate(users.indexer, subgraphDataServiceAddress, delegationTokens, 0);