fix: use official Bun setup action in CI workflow (#131)

- Replace manual Bun installation with oven-sh/setup-bun@v2
- Fixes PATH issue where bun wasn't available in subsequent steps
- Ensures Bun is properly installed before running TypeScript validator

This fixes the issue where the TypeScript validator couldn't run because
Bun wasn't in the PATH. The official setup action handles this correctly.

Related: Fixes CI for PR #128

.github/workflows/validate-registry.yml

@@ -113,10 +113,12 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y jq
-          
-          # Install Bun for TypeScript validator
-          curl -fsSL https://bun.sh/install | bash
-          echo "$HOME/.bun/bin" >> $GITHUB_PATH
+      
+      - name: Install Bun
+        if: github.event.inputs.skip_validation != 'true'
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
       
       - name: Make scripts executable
         if: github.event.inputs.skip_validation != 'true'