fix: stabilize streamed agent text

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/shared/streamingText.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, test } from 'bun:test';
+
+import { mergeStreamingText } from '@shared/utils/streamingText';
+
+describe('streaming text merge', () => {
+  test('appends plain deltas', () => {
+    expect(mergeStreamingText('I am', ' going')).toBe('I am going');
+  });
+
+  test('replaces with a growing snapshot when the incoming text already contains the current text', () => {
+    expect(mergeStreamingText('I am', 'I am going')).toBe('I am going');
+  });
+
+  test('preserves the current text when the incoming update is a duplicate subset', () => {
+    expect(mergeStreamingText('I am going', 'going')).toBe('I am going');
+  });
+
+  test('uses overlap matching to avoid duplicated joins', () => {
+    expect(mergeStreamingText('Hello wor', 'world')).toBe('Hello world');
+  });
+
+  test('replaces with a revised snapshot when the updates share most of the same tokens', () => {
+    const current = 'I mirror the existing button pattern and add brief toggle docs.';
+    const incoming = 'I found the standalone component pattern and I am updating toggle docs next.';
+
+    expect(mergeStreamingText(current, incoming)).toBe(incoming);
+  });
+});