chore: add protoc to tool section (#13709)

Add protoc to tool section in librarian.yaml.

Locally tested `librarian install` and `librarian generate apphub`, the
`protoc` binary is from the librarian managed directory:
```
((.venv) ) joewa@joewa-glinux-medium:~/workspace/google-cloud-java$ go run github.com/googleapis/librarian/cmd/librarian@${V} generate apphub -v
/usr/local/google/home/joewa/.cache/librarian/bin/protoc/v33.2/bin/protoc --experimental_allow_proto3_optional ...
```

Fixes googleapis/librarian#6708

Author: JoeWang1127

librarian.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 language: java
-version: v0.24.1-0.20260707150039-db37cc1a8f59
+version: v0.25.1-0.20260708160859-939ae6f16e82
 repo: googleapis/google-cloud-java
 sources:
   googleapis:
@@ -47,6 +47,9 @@ tools:
     - name: synthtool
       version: ""
       local_path: sdk-platform-java/hermetic_build/library_generation
+  protoc:
+    version: "33.2"
+    sha256: b24b53f87c151bfd48b112fe4c3a6e6574e5198874f38036aff41df3456b8caf
 default:
   java:
     custom_group_ids: