fix(ci): install build-essential before cargo test in ai-credits job

The bare bookworm runner image has no C toolchain at all, unlike
build.yaml's jobs which build inside a Dockerfile-based image with one
already present. proc-macro2/libc/etc build scripts need `cc` to link
and failed immediately (error: linker `cc` not found) before any real
test code ran.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/test.yaml

@@ -48,6 +48,16 @@ jobs:
         run: |
           test -n "${TEST_DATABASE_URL:-}" || { echo "TEST_DATABASE_URL secret is not set - see docs/LIVE_SERVER_RUNBOOK.md step 6"; exit 1; }
 
+      - name: Install build toolchain
+        if: steps.check.outputs.run == 'true'
+        run: |
+          set -euo pipefail
+          # The bare bookworm runner image (unlike build.yaml's Dockerfile-based
+          # builds) has no C toolchain at all - proc-macro2/libc/etc. build
+          # scripts need `cc` to link, which fails immediately without this.
+          apt-get update -qq
+          apt-get install -y -qq build-essential pkg-config
+
       - name: Install Rust
         if: steps.check.outputs.run == 'true'
         run: |