test(desktop): cover undefined/null attachment holes in ref helpers

Regression for the refText crash: attachmentDisplayText and
optimisticAttachmentRef must return null (not throw) when handed an
undefined/null attachment hole, so the submit path can't reproduce
"Cannot read properties of undefined (reading 'refText')".

apps/desktop/src/lib/chat-runtime.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest'
 
 import type { ComposerAttachment } from '@/store/composer'
 
-import { coerceThinkingText, optimisticAttachmentRef, parseCommandDispatch } from './chat-runtime'
+import { attachmentDisplayText, coerceThinkingText, optimisticAttachmentRef, parseCommandDispatch } from './chat-runtime'
 
 const DATA_URL = 'data:image/png;base64,iVBORw0KGgoAAAANS'
 
@@ -36,6 +36,32 @@ describe('optimisticAttachmentRef', () => {
       '@file:src/a.ts'
     )
   })
+
+  // Session switches / draft restores can leave undefined|null holes in the
+  // composer attachments array. AttachmentList already filters them (#49624),
+  // but the submit path maps the same array through these helpers — an unguarded
+  // hole threw "Cannot read properties of undefined (reading 'refText')",
+  // crashing the chat surface (blank pane). The helpers must no-op on holes.
+  it('returns null for an undefined attachment instead of throwing', () => {
+    expect(() => optimisticAttachmentRef(undefined as unknown as ComposerAttachment)).not.toThrow()
+    expect(optimisticAttachmentRef(undefined as unknown as ComposerAttachment)).toBeNull()
+  })
+
+  it('returns null for a null attachment instead of throwing', () => {
+    expect(optimisticAttachmentRef(null as unknown as ComposerAttachment)).toBeNull()
+  })
+})
+
+describe('attachmentDisplayText', () => {
+  it('returns null for undefined|null instead of reading .kind/.refText on a hole', () => {
+    expect(() => attachmentDisplayText(undefined as unknown as ComposerAttachment)).not.toThrow()
+    expect(attachmentDisplayText(undefined as unknown as ComposerAttachment)).toBeNull()
+    expect(attachmentDisplayText(null as unknown as ComposerAttachment)).toBeNull()
+  })
+
+  it('still resolves a normal file ref', () => {
+    expect(attachmentDisplayText(attachment({ kind: 'file', refText: '@file:src/a.ts' }))).toBe('@file:src/a.ts')
+  })
 })
 
 describe('coerceThinkingText', () => {