Add concurrency group to testnet CI to prevent L1 nonce conflicts

Only one testnet deploy can run at a time since they share the same
L1 account. Concurrent runs cause 'replacement transaction underpriced'
errors from nonce collisions on Sepolia.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: critesjosh

.github/workflows/testnet.yaml

@@ -9,6 +9,10 @@ on:
       - testnet
   workflow_dispatch:
 
+concurrency:
+  group: testnet-deploy
+  cancel-in-progress: true
+
 jobs:
   testnet-deploy-account:
     name: Deploy Account to Testnet