feat: add markdown composer backend foundation

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

tests/shared/chatMessage.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, test } from 'bun:test';
+
+import {
+  hasMeaningfulChatMessageContent,
+  normalizeChatMessageLineEndings,
+  prepareChatMessageContent,
+} from '@shared/utils/chatMessage';
+
+describe('chat message helpers', () => {
+  test('normalizes Windows line endings without trimming meaningful markdown', () => {
+    expect(
+      prepareChatMessageContent('```ts\r\n  const answer = 42;\r\n```\r\n'),
+    ).toBe('```ts\n  const answer = 42;\n```\n');
+  });
+
+  test('treats whitespace-only input as empty', () => {
+    expect(hasMeaningfulChatMessageContent(' \r\n\t ')).toBe(false);
+    expect(prepareChatMessageContent(' \r\n\t ')).toBeUndefined();
+  });
+
+  test('normalizes mixed newline sequences consistently', () => {
+    expect(normalizeChatMessageLineEndings('line 1\r\nline 2\rline 3')).toBe(
+      'line 1\nline 2\nline 3',
+    );
+  });
+});