fix: Ensure proper errors are reported when authentication fails (#328)

I can't reproduce the failures mentioned in
#325 so attempting to open
the PR again to see if it fails on CI.

Fixes #317

Author: fjuma

client/base/pom.xml

@@ -48,6 +48,11 @@
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-spec</artifactId>
         </dependency>
+        <dependency>
+            <groupId>${project.groupId}</groupId>
+            <artifactId>a2a-java-sdk-spec-grpc</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter-api</artifactId>
@@ -64,6 +69,16 @@
             <artifactId>slf4j-jdk14</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>io.grpc</groupId>
+            <artifactId>grpc-testing</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>io.grpc</groupId>
+            <artifactId>grpc-inprocess</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>