feat(scripts): require DKG_CONTRACT_ADDRESS in AddKeyperSet

The AddKeyperSet deployment script now reads the DKG contract address
with vm.envAddress("DKG_CONTRACT_ADDRESS") instead of the
vm.envOr(..., address(0)) fallback, and calls setDKGContract
unconditionally. This is the deployment-path complement to the
on-chain addKeyperSet validation added in ef231f8: a zero DKG address
would now revert DKGContractNotSet at the contract call, so failing
earlier at env-resolution time with Foundry's clear "environment
variable not found" message is strictly better than a silent
zero-address branch that always fails downstream.

Key decisions:
- Scope limited to AddKeyperSet.s.sol per the PRD/issue. The byte-
  identical AddKeyperSet.gnosh.s.sol is left untouched; it is not on any
  automated path (the e2e add-keyper-set mise-task always invokes
  AddKeyperSet.s.sol regardless of deployment type), so hardening it is
  a separate, out-of-scope follow-up.
- No new forge test: scripts are not unit-tested in this repo, and the
  new revert-on-missing-env behavior is Foundry framework semantics
  (vm.envAddress), not our logic.

Files changed:
- script/AddKeyperSet.s.sol

Verification: forge clean && forge build clean (only pre-existing
unsafe-typecast lints in scripts/benchmarks); forge test 133/133 pass.
The e2e add-keyper-set task already derives DKG_CONTRACT_ADDRESS from the
deployed DKGContract (both Deploy.service and Deploy.gnosh create one),
so the success path is preserved.

Notes: e2e suite (test-dkg-happy-path, test-dkg-offline-recovery) not
run -- mise is absent and its mise.run | sh bootstrap is blocked by the
sandbox classifier (Code-from-External), as in every prior commit in
this PRD area. The change is a one-line deployment-script edit with no
ABI/runtime impact on correctly-configured keyper sets, so the Go e2e
flows are unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jannikluhn

script/AddKeyperSet.s.sol

@@ -41,7 +41,7 @@ contract AddKeyperSet is Script {
             keyBroadcastContractAddress
         );
 
-        address dkgContract = vm.envOr("DKG_CONTRACT_ADDRESS", address(0));
+        address dkgContract = vm.envAddress("DKG_CONTRACT_ADDRESS");
 
         address[] memory keypers = vm.envAddress("KEYPER_ADDRESSES", ",");
         uint256 threshold = vm.envUint("THRESHOLD");
@@ -59,9 +59,7 @@ contract AddKeyperSet is Script {
         keyperSet.addMembers(keypers);
         keyperSet.setThreshold(uint64(threshold));
         keyperSet.setPublisher(address(eonKeyPublish));
-        if (dkgContract != address(0)) {
-            keyperSet.setDKGContract(dkgContract);
-        }
+        keyperSet.setDKGContract(dkgContract);
         keyperSet.setFinalized();
         console.log("keyperSet:", address(keyperSet));
         console.log("eonKeyPublish:", address(eonKeyPublish));