fix(a2a): simplify card resolution and fix redundant fetching

packages/core/src/agents/a2a-client-manager.test.ts

@@ -60,8 +60,8 @@ describe('A2AClientManager', () => {
 
   beforeEach(() => {
     vi.clearAllMocks();
+    A2AClientManager.resetInstanceForTesting();
     manager = A2AClientManager.getInstance();
-    manager.clearCache();
 
     // Re-create the instances as plain objects that can be spied on
     const factoryInstance = {

packages/core/src/agents/a2a-client-manager.ts

@@ -72,10 +72,19 @@ export class A2AClientManager {
     return A2AClientManager.instance;
   }
 
+  /**
+   * Resets the singleton instance. Only for testing purposes.
+   * @internal
+   */
+  static resetInstanceForTesting() {
+    // @ts-expect-error - Resetting singleton for testing
+    A2AClientManager.instance = undefined;
+  }
+
   /**
    * Loads an agent by fetching its AgentCard and caches the client.
    * @param name The name to assign to the agent.
-   * @param agentCardUrl {string} The full URL to the agent's card.
+   * @param agentCardUrl The full URL to the agent's card.
    * @param authHandler Optional authentication handler to use for this agent.
    * @returns The loaded AgentCard.
    */
@@ -185,7 +194,7 @@ export class A2AClientManager {
     try {
       yield* client.sendMessageStream(messageParams, {
         signal: options?.signal,
-      }) as AsyncIterable<SendMessageResult>;
+      });
     } catch (error: unknown) {
       const prefix = `[A2AClientManager] sendMessageStream Error [${agentName}]`;
       if (error instanceof Error) {