feat(a2a): switch from callback-based to event-driven tool scheduler (#21467)

Co-authored-by: Abhi <abhipatel@google.com>
Co-authored-by: Adam Weidman <adamfweidman@google.com>

packages/a2a-server/src/utils/testing_utils.ts

@@ -64,6 +64,7 @@ export function createMockConfig(
     getEmbeddingModel: vi.fn().mockReturnValue('text-embedding-004'),
     getSessionId: vi.fn().mockReturnValue('test-session-id'),
     getUserTier: vi.fn(),
+    isEventDrivenSchedulerEnabled: vi.fn().mockReturnValue(false),
     getMessageBus: vi.fn(),
     getPolicyEngine: vi.fn(),
     getEnableExtensionReloading: vi.fn().mockReturnValue(false),