ci: run balances tests on every PR, not just schedule/manual dispatch

This is what would have caught the 2026-07-09 HEZ-on-Asset-Hub silent sync
regression at PR time instead of hours into a live-app investigation after
merge. Not yet wired as a required status check - want to confirm it runs
clean on real PRs first (TronBalancesIntegrationTest has a known TronGrid
rate-limit flake that would need addressing before this can safely block
merges).

.github/workflows/balances_test.yml

@@ -1,6 +1,7 @@
 name: Run balances tests
 
 on:
+  pull_request:
   workflow_dispatch:
   schedule:
     - cron: '0 */8 * * *'