feat(ui): attach large text pastes as virtual files

Offer to turn sufficiently large plain-text clipboard pastes into
pasted-context-N.txt attachments instead of inserting them into the
composer, with ask/attach/inline composer settings.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/components/chat/__tests__/attachmentCitations.test.ts

@@ -5,6 +5,7 @@ import {
     buildAttachmentCitationText,
     findAttachmentCitationRanges,
     isGenericImageFilename,
+    nextPastedContextFilename,
 } from '../attachmentCitations';
 
 describe('attachment citations', () => {
@@ -53,4 +54,10 @@ describe('attachment citations', () => {
             ['desktop.jpg'],
         )).toEqual([{ start: 8, end: 21 }]);
     });
+
+    test('assigns sequential pasted-context filenames', () => {
+        expect(nextPastedContextFilename([])).toBe('pasted-context-1.txt');
+        expect(nextPastedContextFilename(['pasted-context-1.txt', 'notes.md'])).toBe('pasted-context-2.txt');
+        expect(nextPastedContextFilename(['PASTED-CONTEXT-2.TXT'])).toBe('pasted-context-1.txt');
+    });
 });