feat(backends): add retry/backoff controls for external CLI execution

src/backends/external.test.ts

@@ -15,6 +15,19 @@ vi.mock('child_process', () => ({
 
 const mockExecFile = vi.mocked(execFile);
 
+function makeFailChild(stderrText = 'failed'): FakeChild {
+  const child = new EventEmitter() as FakeChild;
+  child.stdout = new EventEmitter();
+  child.stderr = new EventEmitter();
+  child.stdin = { end: vi.fn() };
+  child.kill = vi.fn();
+  setImmediate(() => {
+    child.stderr.emit('data', Buffer.from(stderrText));
+    child.emit('close', 1);
+  });
+  return child;
+}
+
 describe('ExternalCliBackend', () => {
   beforeEach(() => {
     vi.clearAllMocks();
@@ -104,4 +117,25 @@ describe('ExternalCliBackend', () => {
     expect(opencode.name).toBe('opencode');
     expect(gemini.name).toBe('gemini');
   });
+
+  it('retries failed backend calls when retries are configured', async () => {
+    spawnMock
+      .mockImplementationOnce(() => makeFailChild('transient'))
+      .mockImplementationOnce(() => makeChild('recovered'));
+    const backend = new ExternalCliBackend({
+      name: 'codex',
+      command: 'codex',
+      retries: 1,
+      retryDelayMs: 0,
+    });
+
+    const result = await backend.process({
+      systemPrompt: 'sys',
+      history: [],
+      message: 'retry me',
+    });
+
+    expect(result).toBe('recovered');
+    expect(spawnMock).toHaveBeenCalledTimes(2);
+  });
 });