fix(ci): pin protoc version in proto-freshness job

Replace apt-get protobuf-compiler (old Ubuntu default) with
arduino/setup-protoc@v3 pinned to v33.2, matching local version.
The version mismatch caused cosmetic diff in generated code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rrodrigu3z

.github/workflows/ci.yml

@@ -46,7 +46,9 @@ jobs:
       - run: pnpm install --frozen-lockfile
 
       - name: Install protoc
-        run: sudo apt-get install -y protobuf-compiler
+        uses: arduino/setup-protoc@v3
+        with:
+          version: "33.2"
 
       - name: Regenerate and check for drift
         run: |