force protobuf version for testing.

Author: zhumin8

pom.xml

@@ -94,7 +94,11 @@
         <artifactId>google-cloud-spanner</artifactId>
         <version>6.55.0</version><!-- {x-version-update:google-cloud-spanner:current} -->
       </dependency>
-
+      <dependency>
+        <groupId>com.google.protobuf</groupId>
+        <artifactId>protobuf-java</artifactId>
+        <version>3.25.5</version>
+      </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-shared-dependencies</artifactId>