fix(worker): don't double-append streamed answer when completed event carries content

apps/agent-worker/tests/unit/turn-outcome.test.ts

@@ -1,6 +1,36 @@
 import { describe, expect, test } from 'vitest';
 
-import { resolveTurnOutcome } from '../../src/runtime/turn-outcome';
+import {
+  resolveTurnOutcome,
+  shouldAppendCompletedContent,
+} from '../../src/runtime/turn-outcome';
+
+describe('shouldAppendCompletedContent', () => {
+  test('does not re-append when deltas already streamed the answer', () => {
+    // Claude/OpenCode emit the final answer as assistant_delta AND as the
+    // content-bearing assistant_completed event. Folding the completed
+    // content in again would produce "<answer><answer>".
+    expect(shouldAppendCompletedContent('the answer', 'the answer')).toBe(
+      false,
+    );
+  });
+
+  test('appends completed content when nothing has streamed yet', () => {
+    expect(shouldAppendCompletedContent('', 'the final result')).toBe(true);
+  });
+
+  test('treats whitespace-only streamed text as empty', () => {
+    expect(shouldAppendCompletedContent('   \n', 'the final result')).toBe(
+      true,
+    );
+  });
+
+  test('does not append when the completed event carries no content', () => {
+    expect(shouldAppendCompletedContent('the answer', undefined)).toBe(false);
+    expect(shouldAppendCompletedContent('', undefined)).toBe(false);
+    expect(shouldAppendCompletedContent('', '')).toBe(false);
+  });
+});
 
 describe('resolveTurnOutcome', () => {
   test('partial streamed text + error is surfaced as failed and text preserved', () => {