fix: consistent token caching and test updates for google-credentials provider

- Cache access tokens alongside ID tokens for consistent retry behavior
- Add targetUrl to registry test assertion after merge
- Add tests for cache hits, 403 retry, and retry exhaustion

packages/core/src/agents/registry.test.ts

@@ -593,6 +593,7 @@ describe('AgentRegistry', () => {
       expect(A2AAuthProviderFactory.create).toHaveBeenCalledWith({
         authConfig: mockAuth,
         agentName: 'RemoteAgentWithAuth',
+        targetUrl: 'https://example.com/card',
         agentCardUrl: 'https://example.com/card',
       });
       expect(loadAgentSpy).toHaveBeenCalledWith(