feat: add default noop providers for AgentCard and -Executor (#215)

This avoids CDI injection problems when building an application with the
Quarkus plugin, and still makes people have to implement them to run
their application

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Fixes #213

Author: kabir

reference-impl/src/main/java/io/a2a/server/apps/quarkus/DefaultProducers.java

@@ -0,0 +1,48 @@
+package io.a2a.server.apps.quarkus;
+
+import java.util.ArrayList;
+import java.util.Collections;
+
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.enterprise.inject.Default;
+import jakarta.ws.rs.Produces;
+
+import io.a2a.server.PublicAgentCard;
+import io.a2a.server.agentexecution.AgentExecutor;
+import io.a2a.server.agentexecution.RequestContext;
+import io.a2a.server.events.EventQueue;
+import io.a2a.spec.AgentCapabilities;
+import io.a2a.spec.AgentCard;
+import io.a2a.spec.JSONRPCError;
+import io.quarkus.arc.DefaultBean;
+
+/**
+ * Contains beans annotated with the Quarkus @DefaultBean annotation, in order to avoid
+ * injection failures when building the Quarkus application as discussed in
+ * <a href="https://github.com/a2aproject/a2a-java/issues/213">Issue 213</a>.
+ *
+ * If an application provides actual implementations of these beans,
+ * those will be used instead.
+ */
+@ApplicationScoped
+public class DefaultProducers {
+    @Produces
+    @PublicAgentCard
+    @DefaultBean
+    public AgentCard createDefaultAgentCard() {
+        throw new IllegalStateException(wrap("Please provide your own AgentCard implementation"));
+    }
+
+    @Produces
+    @Default
+    @DefaultBean
+    public AgentExecutor createDefaultAgentExecutor() {
+        throw new IllegalStateException(wrap("Please provide your own AgentExecutor implementation"));
+    }
+
+    private String wrap(String s) {
+        return s +
+                " as a CDI bean. Your bean will automatically take precedence over this @DefaultBean " +
+                "annotated implementation.";
+    }
+}