fix(ci): Add pgrx init step to benchmark workflow

The benchmark step was failing because pgrx wasn't initialized.
Added cargo-pgrx install and pgrx init steps before running benchmarks.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/postgres-extension-ci.yml

@@ -170,6 +170,13 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y postgresql-16 postgresql-server-dev-16
 
+      - name: Install cargo-pgrx
+        run: cargo install cargo-pgrx --version 0.12.0 --locked
+
+      - name: Initialize pgrx
+        run: cargo pgrx init --pg16=/usr/lib/postgresql/16/bin/pg_config
+        working-directory: crates/ruvector-postgres
+
       - name: Run benchmarks
         run: cargo bench --no-default-features --features pg16 -- --output-format bencher | tee benchmark-output.txt
         working-directory: crates/ruvector-postgres