fix(ci): eliminate duplicate CI runs in tox workflow (#1811)

This change eliminates redundant CI executions by:
- Limiting 'push' triggers to only the 'main' branch for post-merge verification
- Using 'pull_request' for PR validation to run exactly once per commit
- Adding concurrency control to cancel in-progress runs when new commits are pushed

Before: Each PR commit triggered CI twice (push + pull_request events)
After: Each PR commit triggers CI exactly once (pull_request event only)

The workflow still runs on push to main and supports all merge scenarios
while preventing resource waste from duplicate executions.

Author: danceratopz

.github/workflows/tox_verify.yaml

@@ -1,6 +1,14 @@
 name: Tox
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
 
 jobs:
   lint: