Add push event trigger and merge simulation for PR workflow

.github/workflows/pr-main.yml

@@ -4,6 +4,9 @@ on:
   pull_request:
     branches:
       - main
+  push:
+    branches:
+      - main
 
 env:
   CARGO_TERM_COLOR: always
@@ -33,6 +36,12 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Merge PR branch into main
+        if: ${{ github.event_name == 'pull_request'}}
+        run: |
+          git fetch origin ${{ github.event.pull_request.head.ref }}
+          git merge FETCH_HEAD -m "CI Merge Simulation"
+
       - name: Set up Rust
         uses: actions-rust-lang/setup-rust-toolchain@v1